HI, updating from 17.01.4 to 18.06 RC1 also I have the same problem with the DIR-860L and Xiaomi 3G.
Sun Jun 24 22:36:37 2018 daemon.notice netifd: Interface 'wan' is now down
Sun Jun 24 22:36:37 2018 daemon.notice netifd: Interface 'wan' is setting up now
Sun Jun 24 22:36:38 2018 daemon.err insmod: module is already loaded - slhc
Sun Jun 24 22:36:38 2018 daemon.err insmod: module is already loaded - ppp_generic
Sun Jun 24 22:36:38 2018 daemon.err insmod: module is already loaded - pppox
Sun Jun 24 22:36:38 2018 daemon.err insmod: module is already loaded - pppoe
Sun Jun 24 22:36:38 2018 daemon.info dnsmasq[3452]: read /etc/hosts - 4 addresses
Sun Jun 24 22:36:38 2018 daemon.info dnsmasq[3452]: read /tmp/hosts/odhcpd - 1 addresses
Sun Jun 24 22:36:38 2018 daemon.info dnsmasq[3452]: read /tmp/hosts/dhcp.cfg01411c - 3 addresses
Sun Jun 24 22:36:38 2018 daemon.info dnsmasq-dhcp[3452]: read /etc/ethers - 0 addresses
Sun Jun 24 22:36:38 2018 daemon.info pppd[6291]: Plugin rp-pppoe.so loaded.
Sun Jun 24 22:36:38 2018 daemon.info pppd[6291]: RP-PPPoE plugin version 3.8p compiled against pppd 2.4.7
Sun Jun 24 22:36:38 2018 daemon.notice pppd[6291]: pppd 2.4.7 started by root, uid 0
Sun Jun 24 22:36:53 2018 daemon.warn pppd[6291]: Timeout waiting for PADO packets
Sun Jun 24 22:36:53 2018 daemon.err pppd[6291]: Unable to complete PPPoE Discovery
Sun Jun 24 22:36:53 2018 daemon.info pppd[6291]: Exit.
Sun Jun 24 22:36:53 2018 daemon.notice netifd: Interface 'wan' is now down
Sun Jun 24 22:36:53 2018 daemon.notice netifd: Interface 'wan' is setting up now
Sun Jun 24 22:36:53 2018 daemon.err insmod: module is already loaded - slhc
Sun Jun 24 22:36:53 2018 daemon.err insmod: module is already loaded - ppp_generic
Sun Jun 24 22:36:53 2018 daemon.err insmod: module is already loaded - pppox
Sun Jun 24 22:36:53 2018 daemon.err insmod: module is already loaded - pppoe
Sun Jun 24 22:36:53 2018 daemon.info pppd[6494]: Plugin rp-pppoe.so loaded.
Sun Jun 24 22:36:53 2018 daemon.info pppd[6494]: RP-PPPoE plugin version 3.8p compiled against pppd 2.4.7
Sun Jun 24 22:36:53 2018 daemon.notice pppd[6494]: pppd 2.4.7 started by root, uid 0
Sun Jun 24 22:37:08 2018 daemon.warn pppd[6494]: Timeout waiting for PADO packets
Sun Jun 24 22:37:08 2018 daemon.err pppd[6494]: Unable to complete PPPoE Discovery
Sun Jun 24 22:37:08 2018 daemon.info pppd[6494]: Exit.
Sun Jun 24 22:37:08 2018 daemon.notice netifd: Interface 'wan' is now down
Sun Jun 24 22:37:08 2018 daemon.notice netifd: Interface 'wan' is setting up now
Sun Jun 24 22:37:08 2018 daemon.err insmod: module is already loaded - slhc
Sun Jun 24 22:37:08 2018 daemon.err insmod: module is already loaded - ppp_generic
Sun Jun 24 22:37:08 2018 daemon.err insmod: module is already loaded - pppox
Sun Jun 24 22:37:08 2018 daemon.err insmod: module is already loaded - pppoe
Sun Jun 24 22:37:09 2018 daemon.info pppd[6666]: Plugin rp-pppoe.so loaded.
Sun Jun 24 22:37:09 2018 daemon.info pppd[6666]: RP-PPPoE plugin version 3.8p compiled against pppd 2.4.7
Sun Jun 24 22:37:09 2018 daemon.notice pppd[6666]: pppd 2.4.7 started by root, uid 0
Sun Jun 24 22:37:24 2018 daemon.warn pppd[6666]: Timeout waiting for PADO packets
Sun Jun 24 22:37:24 2018 daemon.err pppd[6666]: Unable to complete PPPoE Discovery
Sun Jun 24 22:37:24 2018 daemon.info pppd[6666]: Exit.
Sun Jun 24 22:37:24 2018 daemon.notice netifd: Interface 'wan' is now down
Sun Jun 24 22:37:24 2018 daemon.notice netifd: Interface 'wan' is setting up now
Sun Jun 24 22:37:24 2018 daemon.err insmod: module is already loaded - slhc
Sun Jun 24 22:37:24 2018 daemon.err insmod: module is already loaded - ppp_generic
Sun Jun 24 22:37:24 2018 daemon.err insmod: module is already loaded - pppox
Sun Jun 24 22:37:24 2018 daemon.err insmod: module is already loaded - pppoe
Sun Jun 24 22:37:24 2018 daemon.info pppd[6799]: Plugin rp-pppoe.so loaded.
Sun Jun 24 22:37:24 2018 daemon.info pppd[6799]: RP-PPPoE plugin version 3.8p compiled against pppd 2.4.7
Sun Jun 24 22:37:24 2018 daemon.notice pppd[6799]: pppd 2.4.7 started by root, uid 0
Sun Jun 24 22:37:39 2018 daemon.warn pppd[6799]: Timeout waiting for PADO packets
Sun Jun 24 22:37:39 2018 daemon.err pppd[6799]: Unable to complete PPPoE Discovery
Sun Jun 24 22:37:39 2018 daemon.info pppd[6799]: Exit.
Sun Jun 24 22:37:39 2018 daemon.notice netifd: Interface 'wan' is now down
Sun Jun 24 22:37:39 2018 daemon.notice netifd: Interface 'wan' is setting up now
Sun Jun 24 22:37:39 2018 daemon.err insmod: module is already loaded - slhc
Sun Jun 24 22:37:39 2018 daemon.err insmod: module is already loaded - ppp_generic
Sun Jun 24 22:37:39 2018 daemon.err insmod: module is already loaded - pppox
Sun Jun 24 22:37:39 2018 daemon.err insmod: module is already loaded - pppoe
Sun Jun 24 22:37:39 2018 daemon.info pppd[6909]: Plugin rp-pppoe.so loaded.
Sun Jun 24 22:37:39 2018 daemon.info pppd[6909]: RP-PPPoE plugin version 3.8p compiled against pppd 2.4.7
Sun Jun 24 22:37:40 2018 daemon.notice pppd[6909]: pppd 2.4.7 started by root, uid 0
Sun Jun 24 22:37:55 2018 daemon.warn pppd[6909]: Timeout waiting for PADO packets
Sun Jun 24 22:37:55 2018 daemon.err pppd[6909]: Unable to complete PPPoE Discovery
Sun Jun 24 22:37:55 2018 daemon.info pppd[6909]: Exit.
Sun Jun 24 22:37:55 2018 daemon.notice netifd: Interface 'wan' is now down
Sun Jun 24 22:37:55 2018 daemon.notice netifd: Interface 'wan' is setting up now
Sun Jun 24 22:37:55 2018 daemon.err insmod: module is already loaded - slhc
Sun Jun 24 22:37:55 2018 daemon.err insmod: module is already loaded - ppp_generic
Sun Jun 24 22:37:55 2018 daemon.err insmod: module is already loaded - pppox
Sun Jun 24 22:37:55 2018 daemon.err insmod: module is already loaded - pppoe
Sun Jun 24 22:37:55 2018 daemon.info pppd[7086]: Plugin rp-pppoe.so loaded.
Sun Jun 24 22:37:55 2018 daemon.info pppd[7086]: RP-PPPoE plugin version 3.8p compiled against pppd 2.4.7
Sun Jun 24 22:37:55 2018 daemon.notice pppd[7086]: pppd 2.4.7 started by root, uid 0
Sun Jun 24 22:38:00 2018 daemon.info pppd[7086]: PPP session is 26266
Sun Jun 24 22:38:00 2018 daemon.warn pppd[7086]: Connected to 08:96:ad:6b:74:d0 via interface eth0.2
Sun Jun 24 22:38:00 2018 kern.info kernel: [ 353.525389] pppoe-wan: renamed from ppp0
Sun Jun 24 22:38:00 2018 daemon.info pppd[7086]: Using interface pppoe-wan
Sun Jun 24 22:38:00 2018 daemon.notice pppd[7086]: Connect: pppoe-wan <--> eth0.2
Sun Jun 24 22:38:03 2018 daemon.info pppd[7086]: CHAP authentication succeeded
Sun Jun 24 22:38:03 2018 daemon.notice pppd[7086]: CHAP authentication succeeded
Sun Jun 24 22:38:03 2018 daemon.notice pppd[7086]: peer from calling number 08:96:AD:6B:74:D0 authorized
Sun Jun 24 22:38:03 2018 daemon.notice pppd[7086]: local IP address 94.35.xxx.xxx
Sun Jun 24 22:38:03 2018 daemon.notice pppd[7086]: remote IP address 94.32.xxx.xxx
Sun Jun 24 22:38:03 2018 daemon.notice pppd[7086]: primary DNS address 213.205.32.70
Sun Jun 24 22:38:03 2018 daemon.notice pppd[7086]: secondary DNS address 213.205.36.70
Sun Jun 24 22:38:04 2018 daemon.notice netifd: Network device 'pppoe-wan' link is up
Sun Jun 24 22:38:04 2018 daemon.notice netifd: Interface 'wan' is now up
Does anyone have the same problem with 18.06 RC1?