DHCP client udhcpc on WAN port not working

I have installed OpenWrt 19.07.3 in Mikrotik RouterBoard RB750-r2. Using 'dhcp' proto on the wan port does not work properly. If I use another OpenWrt as the upstream router then it receives and sets IPv4 address to the wan port properly.

But when I connect the WAN port of the OpenWrt to the LAN of a Tenda router (active in my house) it (OpenWrt) cannot set the IP address. It seems udhcpc ignores dhcp offers from the Tenda router.

I ran the following commands:

reset; \
NET_IF="wan"; \
NET_DEV="$(uci get network.${NET_IF}.ifname)"; \
ifup "${NET_IF}"; sleep 10; \
tcpdump -v -n -i "${NET_DEV}" udp port 67 or udp port 68 & \
ip address show dev "${NET_DEV}"; \
kill -SIGUSR2 "$(pidof udhcpc)"; sleep 10; \
ip address show dev "${NET_DEV}"; \
kill -SIGUSR1 "$(pidof udhcpc)"; sleep 10; \
ip address show dev "${NET_DEV}"; \
killall tcpdump

and got the following output:

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP100> mtu 1500 qdisc fq_codel state UP qlen 1000
    link/ether cc:2d:e0:69:32:e0 brd ff:ff:ff:ff:ff:ff
    inet6 fe80::ce2d:e0ff:fe69:32e0/64 scope link 
       valid_lft forever preferred_lft forever
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP100> mtu 1500 qdisc fq_codel state UP qlen 1000
    link/ether cc:2d:e0:69:32:e0 brd ff:ff:ff:ff:ff:ff
    inet6 fe80::ce2d:e0ff:fe69:32e0/64 scope link 
       valid_lft forever preferred_lft forever
18:34:41.986525 IP (tos 0x0, ttl 64, id 0, offset 0, flags [none], proto UDP (17), length 328)
    0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from cc:2d:e0:69:32:e0, length 300, xid 0x5675a9b2, Flags [none]
          Client-Ethernet-Address cc:2d:e0:69:32:e0
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Discover
            MSZ Option 57, length 2: 576
            Parameter-Request Option 55, length 8: 
              Subnet-Mask, Default-Gateway, Domain-Name-Server, Hostname
              Domain-Name, BR, NTP, Classless-Static-Route
            Vendor-Class Option 60, length 12: "udhcp 1.30.1"
            Hostname Option 12, length 7: "OpenWrt"
18:34:41.987144 IP (tos 0x0, ttl 64, id 0, offset 0, flags [none], proto UDP (17), length 328)
    0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from cc:2d:e0:69:32:e0, length 300, xid 0x2f5826d, Flags [none]
          Client-Ethernet-Address cc:2d:e0:69:32:e0
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Discover
            Client-ID Option 61, length 7: ether cc:2d:e0:69:32:e0
            Requested-IP Option 50, length 4: 192.168.1.120
            MSZ Option 57, length 2: 576
            Parameter-Request Option 55, length 7: 
              Subnet-Mask, Default-Gateway, Domain-Name-Server, Hostname
              Domain-Name, BR, NTP
            Vendor-Class Option 60, length 12: "udhcp 1.30.1"
18:34:42.982658 IP (tos 0x0, ttl 64, id 0, offset 0, flags [none], proto UDP (17), length 768)
    192.168.0.1.67 > 255.255.255.255.68: BOOTP/DHCP, Reply, length 740, xid 0x5675a9b2, Flags [none]
          Your-IP 192.168.0.107
          Client-Ethernet-Address cc:2d:e0:69:32:e0
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Offer
            Server-ID Option 54, length 4: 192.168.0.1
            Lease-Time Option 51, length 4: 48
            Subnet-Mask Option 1, length 4: 255.255.255.0
            Default-Gateway Option 3, length 4: 192.168.0.1
            Domain-Name-Server Option 6, length 4: 192.168.0.1
            Domain-Name Option 15, length 17: "www.tendawifi.com"
18:34:43.984307 IP (tos 0x0, ttl 64, id 0, offset 0, flags [none], proto UDP (17), length 768)
    192.168.0.1.67 > 255.255.255.255.68: BOOTP/DHCP, Reply, length 740, xid 0x2f5826d, Flags [none]
          Your-IP 192.168.0.107
          Client-Ethernet-Address cc:2d:e0:69:32:e0
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Offer
            Server-ID Option 54, length 4: 192.168.0.1
            Lease-Time Option 51, length 4: 59
            Subnet-Mask Option 1, length 4: 255.255.255.0
            Default-Gateway Option 3, length 4: 192.168.0.1
            Domain-Name-Server Option 6, length 4: 192.168.0.1
            Domain-Name Option 15, length 17: "www.tendawifi.com"
18:34:44.986340 IP (tos 0x0, ttl 64, id 0, offset 0, flags [none], proto UDP (17), length 328)
    0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from cc:2d:e0:69:32:e0, length 300, xid 0x2f5826d, secs 3, Flags [none]
          Client-Ethernet-Address cc:2d:e0:69:32:e0
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Discover
            Client-ID Option 61, length 7: ether cc:2d:e0:69:32:e0
            Requested-IP Option 50, length 4: 192.168.1.120
            MSZ Option 57, length 2: 576
            Parameter-Request Option 55, length 7: 
              Subnet-Mask, Default-Gateway, Domain-Name-Server, Hostname
              Domain-Name, BR, NTP
            Vendor-Class Option 60, length 12: "udhcp 1.30.1"
18:34:44.986755 IP (tos 0x0, ttl 64, id 0, offset 0, flags [none], proto UDP (17), length 328)
    0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from cc:2d:e0:69:32:e0, length 300, xid 0x5675a9b2, secs 3, Flags [none]
          Client-Ethernet-Address cc:2d:e0:69:32:e0
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Discover
            MSZ Option 57, length 2: 576
            Parameter-Request Option 55, length 8: 
              Subnet-Mask, Default-Gateway, Domain-Name-Server, Hostname
              Domain-Name, BR, NTP, Classless-Static-Route
            Vendor-Class Option 60, length 12: "udhcp 1.30.1"
            Hostname Option 12, length 7: "OpenWrt"
18:34:45.979429 IP (tos 0x0, ttl 64, id 0, offset 0, flags [none], proto UDP (17), length 768)
    192.168.0.1.67 > 255.255.255.255.68: BOOTP/DHCP, Reply, length 740, xid 0x2f5826d, Flags [none]
          Your-IP 192.168.0.107
          Client-Ethernet-Address cc:2d:e0:69:32:e0
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Offer
            Server-ID Option 54, length 4: 192.168.0.1
            Lease-Time Option 51, length 4: 58
            Subnet-Mask Option 1, length 4: 255.255.255.0
            Default-Gateway Option 3, length 4: 192.168.0.1
            Domain-Name-Server Option 6, length 4: 192.168.0.1
            Domain-Name Option 15, length 17: "www.tendawifi.com"
18:34:46.972516 IP (tos 0x0, ttl 64, id 0, offset 0, flags [none], proto UDP (17), length 768)
    192.168.0.1.67 > 255.255.255.255.68: BOOTP/DHCP, Reply, length 740, xid 0x5675a9b2, Flags [none]
          Your-IP 192.168.0.107
          Client-Ethernet-Address cc:2d:e0:69:32:e0
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Offer
            Server-ID Option 54, length 4: 192.168.0.1
            Lease-Time Option 51, length 4: 59
            Subnet-Mask Option 1, length 4: 255.255.255.0
            Default-Gateway Option 3, length 4: 192.168.0.1
            Domain-Name-Server Option 6, length 4: 192.168.0.1
            Domain-Name Option 15, length 17: "www.tendawifi.com"
18:34:47.974494 IP (tos 0x0, ttl 64, id 0, offset 0, flags [none], proto UDP (17), length 328)
    0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from cc:2d:e0:69:32:e0, length 300, xid 0x2f5826d, secs 6, Flags [none]
          Client-Ethernet-Address cc:2d:e0:69:32:e0
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Discover
            Client-ID Option 61, length 7: ether cc:2d:e0:69:32:e0
            Requested-IP Option 50, length 4: 192.168.1.120
            MSZ Option 57, length 2: 576
            Parameter-Request Option 55, length 7: 
              Subnet-Mask, Default-Gateway, Domain-Name-Server, Hostname
              Domain-Name, BR, NTP
            Vendor-Class Option 60, length 12: "udhcp 1.30.1"
18:34:47.974948 IP (tos 0x0, ttl 64, id 0, offset 0, flags [none], proto UDP (17), length 328)
    0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from cc:2d:e0:69:32:e0, length 300, xid 0x5675a9b2, secs 6, Flags [none]
          Client-Ethernet-Address cc:2d:e0:69:32:e0
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Discover
            MSZ Option 57, length 2: 576
            Parameter-Request Option 55, length 8: 
              Subnet-Mask, Default-Gateway, Domain-Name-Server, Hostname
              Domain-Name, BR, NTP, Classless-Static-Route
            Vendor-Class Option 60, length 12: "udhcp 1.30.1"
            Hostname Option 12, length 7: "OpenWrt"
18:34:48.972408 IP (tos 0x0, ttl 64, id 0, offset 0, flags [none], proto UDP (17), length 768)
    192.168.0.1.67 > 255.255.255.255.68: BOOTP/DHCP, Reply, length 740, xid 0x2f5826d, Flags [none]
          Your-IP 192.168.0.107
          Client-Ethernet-Address cc:2d:e0:69:32:e0
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Offer
            Server-ID Option 54, length 4: 192.168.0.1
            Lease-Time Option 51, length 4: 58
            Subnet-Mask Option 1, length 4: 255.255.255.0
            Default-Gateway Option 3, length 4: 192.168.0.1
            Domain-Name-Server Option 6, length 4: 192.168.0.1
            Domain-Name Option 15, length 17: "www.tendawifi.com"
18:34:49.972470 IP (tos 0x0, ttl 64, id 0, offset 0, flags [none], proto UDP (17), length 768)
    192.168.0.1.67 > 255.255.255.255.68: BOOTP/DHCP, Reply, length 740, xid 0x5675a9b2, Flags [none]
          Your-IP 192.168.0.107
          Client-Ethernet-Address cc:2d:e0:69:32:e0
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Offer
            Server-ID Option 54, length 4: 192.168.0.1
            Lease-Time Option 51, length 4: 59
            Subnet-Mask Option 1, length 4: 255.255.255.0
            Default-Gateway Option 3, length 4: 192.168.0.1
            Domain-Name-Server Option 6, length 4: 192.168.0.1
            Domain-Name Option 15, length 17: "www.tendawifi.com"
18:34:50.975076 IP (tos 0x0, ttl 64, id 0, offset 0, flags [none], proto UDP (17), length 328)
    0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from cc:2d:e0:69:32:e0, length 300, xid 0x5675a9b2, secs 9, Flags [none]
          Client-Ethernet-Address cc:2d:e0:69:32:e0
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Discover
            MSZ Option 57, length 2: 576
            Parameter-Request Option 55, length 8: 
              Subnet-Mask, Default-Gateway, Domain-Name-Server, Hostname
              Domain-Name, BR, NTP, Classless-Static-Route
            Vendor-Class Option 60, length 12: "udhcp 1.30.1"
            Hostname Option 12, length 7: "OpenWrt"
18:34:50.976844 IP (tos 0x0, ttl 64, id 0, offset 0, flags [none], proto UDP (17), length 768)
    192.168.0.1.67 > 255.255.255.255.68: BOOTP/DHCP, Reply, length 740, xid 0x5675a9b2, Flags [none]
          Your-IP 192.168.0.107
          Client-Ethernet-Address cc:2d:e0:69:32:e0
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Offer
            Server-ID Option 54, length 4: 192.168.0.1
            Lease-Time Option 51, length 4: 59
            Subnet-Mask Option 1, length 4: 255.255.255.0
            Default-Gateway Option 3, length 4: 192.168.0.1
            Domain-Name-Server Option 6, length 4: 192.168.0.1
            Domain-Name Option 15, length 17: "www.tendawifi.com"
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP100> mtu 1500 qdisc fq_codel state UP qlen 1000
    link/ether cc:2d:e0:69:32:e0 brd ff:ff:ff:ff:ff:ff
    inet6 fe80::ce2d:e0ff:fe69:32e0/64 scope link 
       valid_lft forever preferred_lft forever

14 packets captured
14 packets received by filter
0 packets dropped by kernel
[1]+  Done                       tcpdump -v -n -i "${NET_DEV}" udp port 67 or udp port 68

Edit:
Output of the same command using an OpenWrt router as upstream:

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP qlen 1000
    link/ether cc:2d:e0:69:32:e0 brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.120/24 brd 192.168.1.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::ce2d:e0ff:fe69:32e0/64 scope link 
       valid_lft forever preferred_lft forever
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
09:16:28.270475 IP (tos 0x0, ttl 64, id 46849, offset 0, flags [DF], proto UDP (17), length 328)
    192.168.1.120.68 > 192.168.1.1.67: BOOTP/DHCP, Request from cc:2d:e0:69:32:e0, length 300, xid 0x59b2fa68, secs 10, Flags [none]
          Client-IP 192.168.1.120
          Client-Ethernet-Address cc:2d:e0:69:32:e0
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Release
            Server-ID Option 54, length 4: 192.168.1.1
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP qlen 1000
    link/ether cc:2d:e0:69:32:e0 brd ff:ff:ff:ff:ff:ff
    inet6 fe80::ce2d:e0ff:fe69:32e0/64 scope link 
       valid_lft forever preferred_lft forever
09:16:38.309427 IP (tos 0x0, ttl 64, id 0, offset 0, flags [none], proto UDP (17), length 328)
    0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from cc:2d:e0:69:32:e0, length 300, xid 0x661e2c41, Flags [none]
          Client-Ethernet-Address cc:2d:e0:69:32:e0
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Discover
            Requested-IP Option 50, length 4: 192.168.1.120
            MSZ Option 57, length 2: 576
            Parameter-Request Option 55, length 8: 
              Subnet-Mask, Default-Gateway, Domain-Name-Server, Hostname
              Domain-Name, BR, NTP, Classless-Static-Route
            Vendor-Class Option 60, length 12: "udhcp 1.30.1"
            Hostname Option 12, length 7: "OpenWrt"
09:16:38.311421 IP (tos 0x0, ttl 64, id 25537, offset 0, flags [none], proto UDP (17), length 328)
    192.168.1.1.67 > 192.168.1.120.68: BOOTP/DHCP, Reply, length 300, xid 0x661e2c41, Flags [none]
          Your-IP 192.168.1.120
          Server-IP 192.168.1.1
          Client-Ethernet-Address cc:2d:e0:69:32:e0
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Offer
            Server-ID Option 54, length 4: 192.168.1.1
            Lease-Time Option 51, length 4: 43200
            RN Option 58, length 4: 21600
            RB Option 59, length 4: 37800
            Subnet-Mask Option 1, length 4: 255.255.255.0
            BR Option 28, length 4: 192.168.1.255
            Default-Gateway Option 3, length 4: 192.168.1.1
            Domain-Name-Server Option 6, length 4: 192.168.1.1
            Domain-Name Option 15, length 3: "lan"
09:16:38.313418 IP (tos 0x0, ttl 64, id 0, offset 0, flags [none], proto UDP (17), length 328)
    0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from cc:2d:e0:69:32:e0, length 300, xid 0x661e2c41, Flags [none]
          Client-Ethernet-Address cc:2d:e0:69:32:e0
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Request
            Requested-IP Option 50, length 4: 192.168.1.120
            Server-ID Option 54, length 4: 192.168.1.1
            MSZ Option 57, length 2: 576
            Parameter-Request Option 55, length 8: 
              Subnet-Mask, Default-Gateway, Domain-Name-Server, Hostname
              Domain-Name, BR, NTP, Classless-Static-Route
            Vendor-Class Option 60, length 12: "udhcp 1.30.1"
            Hostname Option 12, length 7: "OpenWrt"
09:16:38.316374 IP (tos 0x0, ttl 64, id 25538, offset 0, flags [none], proto UDP (17), length 334)
    192.168.1.1.67 > 192.168.1.120.68: BOOTP/DHCP, Reply, length 306, xid 0x661e2c41, Flags [none]
          Your-IP 192.168.1.120
          Server-IP 192.168.1.1
          Client-Ethernet-Address cc:2d:e0:69:32:e0
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: ACK
            Server-ID Option 54, length 4: 192.168.1.1
            Lease-Time Option 51, length 4: 43200
            RN Option 58, length 4: 21600
            RB Option 59, length 4: 37800
            Subnet-Mask Option 1, length 4: 255.255.255.0
            BR Option 28, length 4: 192.168.1.255
            Default-Gateway Option 3, length 4: 192.168.1.1
            Domain-Name-Server Option 6, length 4: 192.168.1.1
            Domain-Name Option 15, length 3: "lan"
            Hostname Option 12, length 7: "OpenWrt"
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP qlen 1000
    link/ether cc:2d:e0:69:32:e0 brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.120/24 brd 192.168.1.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::ce2d:e0ff:fe69:32e0/64 scope link 
       valid_lft forever preferred_lft forever

5 packets captured
5 packets received by filter
0 packets dropped by kernel
[1]-  Done                       tcpdump -v -n -i "${NET_DEV}" udp port 67 or udp port 68

It seems that the format/contents of the DHCP-offer message of Tenda is different from that of OpenWrt.

Note: The OpenWrt (testing router) is configured to use 192.168.10.0/24 subnet because upstream uses the default.