@Ethan-TP Of course, I tried using a different browser and a different computer. I turned WAN off and on several times. I restarted the router. I reconfigured the WAN network. For example, when...

There was no need for this before, so this is the first backup configuration. The USB modem port can be turned on and off, but it is not visible.

I want to set up WAN/LAN3 as a backup WAN in case the main WAN fails, but there is no option to select WAN/LAN3 in the backup WAN drop-down menu. I am attaching screenshots for reference. I found a...