OK, now it is clear.
I alreadi have the two host with IP 172.21.1.71 and 172.21.1.72 - do I need also to modify the network file like below?
under WAN in /etc/config/network
list ipaddr '172.21.1.71/32'
list ipaddr '172.21.1.72/32'
Below the actual Firewall configuration
package firewall
config defaults
option input 'ACCEPT'
option output 'ACCEPT'
option synflood_protect '1'
option forward 'ACCEPT'
config zone
option name 'lan'
option input 'ACCEPT'
option output 'ACCEPT'
option forward 'ACCEPT'
list network 'lan'
option masq '1'
config zone
option name 'wan'
option output 'ACCEPT'
option mtu_fix '1'
list network 'WAN'
option input 'ACCEPT'
option forward 'ACCEPT'
option masq '1'
config forwarding
option src 'lan'
option dest 'wan'
config rule
option name 'Allow-DHCP-Renew'
option src 'wan'
option proto 'udp'
option dest_port '68'
option target 'ACCEPT'
option family 'ipv4'
config rule
option name 'Allow-Ping'
option src 'wan'
option proto 'icmp'
option icmp_type 'echo-request'
option family 'ipv4'
option target 'ACCEPT'
config rule
option name 'Allow-IGMP'
option src 'wan'
option proto 'igmp'
option family 'ipv4'
option target 'ACCEPT'
config rule
option name 'Allow-DHCPv6'
option src 'wan'
option proto 'udp'
option dest_port '546'
option family 'ipv6'
option target 'ACCEPT'
config rule
option name 'Allow-MLD'
option src 'wan'
option proto 'icmp'
option src_ip 'fe80::/10'
list icmp_type '130/0'
list icmp_type '131/0'
list icmp_type '132/0'
list icmp_type '143/0'
option family 'ipv6'
option target 'ACCEPT'
config rule
option name 'Allow-ICMPv6-Input'
option src 'wan'
option proto 'icmp'
list icmp_type 'echo-request'
list icmp_type 'echo-reply'
list icmp_type 'destination-unreachable'
list icmp_type 'packet-too-big'
list icmp_type 'time-exceeded'
list icmp_type 'bad-header'
list icmp_type 'unknown-header-type'
list icmp_type 'router-solicitation'
list icmp_type 'neighbour-solicitation'
list icmp_type 'router-advertisement'
list icmp_type 'neighbour-advertisement'
option limit '1000/sec'
option family 'ipv6'
option target 'ACCEPT'
config rule
option name 'Allow-ICMPv6-Forward'
option src 'wan'
option dest '*'
option proto 'icmp'
list icmp_type 'echo-request'
list icmp_type 'echo-reply'
list icmp_type 'destination-unreachable'
list icmp_type 'packet-too-big'
list icmp_type 'time-exceeded'
list icmp_type 'bad-header'
list icmp_type 'unknown-header-type'
option limit '1000/sec'
option family 'ipv6'
option target 'ACCEPT'
config rule
option name 'Allow-IPSec-ESP'
option src 'wan'
option dest 'lan'
option proto 'esp'
option target 'ACCEPT'
config rule
option name 'Allow-ISAKMP'
option src 'wan'
option dest 'lan'
option dest_port '500'
option proto 'udp'
option target 'ACCEPT'
config forwarding
option src 'wan'
option dest 'lan'
config redirect
option target 'DNAT'
option src 'wan'
option src_ip '172.20.0.0/16'
option src_dip '172.20.1.71'
option name 'REDIRECT'
option dest_ip '172.21.1.71'
option family 'ipv4'
option dest 'lan'
config redirect
option target 'DNAT'
option src 'wan'
option src_ip '172.20.0.0/16'
option src_dip '172.20.1.72'
option name 'REDIRECT'
option dest_ip '172.21.1.72'
option family 'ipv4'
option dest 'lan'
Below actually Network configuration
config interface 'loopback'
option device 'lo'
option proto 'static'
option ipaddr '127.0.0.1'
option netmask '255.0.0.0'
config globals 'globals'
option ula_prefix 'fd44:f7df:148b::/48'
config device
option name 'br-lan'
option type 'bridge'
list ports 'eth0'
config interface 'lan'
option device 'br-lan'
option proto 'static'
option ip6assign '60'
list ipaddr '172.21.1.61/16'
option gateway '172.21.1.254'
list dns '208.67.222.222'
config interface 'WAN'
option proto 'static'
option device 'eth1'
list ipaddr '172.20.1.61/16'