I tried your scenario on my side and i was able to make it work :)
How did i configure it:
1. Updated the router to the latest firmware with wiping all configuration so that it is like a fresh one.
2. Login to the Web UI and configured the settings like time,password,lan,wifi and rms.
3. Now I went to Network > WAN then manually turned off Wired WAN save and apply like the image below:
4. Now I open putty vi /etc/config/network then edit on the lan interface option ifname "eth0 eth1".
It should be like this:
then /etc/init.d/network restart
5. Then after that i inserted the sim card and rebooted the router.
Hope it helps :)