Hi everyone
sorry for my english.

I have a problem to solve, I use a Unifi UI infrastructure, I have a FTTH connection connected to WAN1 in pppoe, and everything works.
I purchased a TL-MR110 to have a backup 4G connection in case the main WAN1 network goes offline. I have a port forwarding set up in my router (UI) which by calling port 12345 refers to internal port 80. I would like to do the same thing with WAN2 (TL-MR110) however it takes the router's DHCP IP. Is there a way to make this transition between the two routers or give the WAN2 on my main router a pppoe connection so as to directly take the public IP provided by the 4g?

Thank you very much

I hope I have explained myself. I attach some photos (I obscured the public IP of WAN1)