It looks like the router has Firmware 4 and I do not know what produces the “No Internet Connection! Find a network to reconnect” message in that version. carrier-monitor is different, so you should not disable it. Have you tried disabling and stopping mwan3?
I am not certain what you mean by the above, but LAN IPv4 address has to be a new IP on the same subnet as your main Internet router, LAN IPv4 gateway has to be the IP address of your main Internet router and LAN Use custom DNS servers can just be 8.8.8.8 temporarily for testing.