Hello,
Is the issue still relevant?
Have you found a solution?
I am not particularly familiar wit the WebRTC protocol, however, it seems that for hosts behind NAT, which it likely is in your case, some TURN server is required to establish connection.
Also, I assume, for the connectivity to work, a public IP needs to be assigned for the router. Based on the troubleshoot file, only one of the modems has an address routable over the internet, the other is assigned with a CG - NATed address.
As for ports, for testing purposes, you can simply open the required range of UDP ports from the WebUI Network -> Firewall -> Traffic rules section.
Best regards,