Hello,
I have the same issues. I have a setup, where my wired WAN is the primary, fallback to Wifi (client) WAN, and thirdly to mobile SIM WAN.
This setup worked fine for about 3 months or more ... and then suddenly, without any config change or so, the Wired WAN disappears from the list... It cannot be selected, and even, as there is a physical and working network cable in the WAN port, the image displays it as not connected - though the traffic light of the WAN port flickers...
I had the same problems in the beginning when i started using the modem. Then I had to factory reset it, and do the whole configuration again !
Now again out of the blue, the problem is back....
It is very annoying because I haven't touched the config in months !!!
Edit:
1 - I tried the command suggested earlier in this thread, then the wired interface appears again, but one of the SIM mobile WAN's has disappeared. And also, the WAN does not come up (no dhcp address). -> i had to switch between static and dhcp config to actually "kick it back to life..."
2 - now all of a sudden my SIM WAN mobile connection is having problems... I had to restart the modem a couple of times...
3 - it keeps telling me there is a firmware upgrade, but I already installed it twice (it has the same version !!!) and still keeps alerting me it has an update....