I was working with a Roon user on a double NAT issue preventing ARC from functioning. They have Verizon FiOS service which is supplied with a G3100 router.
This router is only needed if you have FiOS TV. Because the router is required for FiOS TV it will not properly go into Bridge mode. However, if you don’t need FiOS TV take it out of your network.
Plug your own router directly into the ONT. Like this:
ONT -> (wan) |_your_ router| (lan) ---- \ ---> roon and other stuff
This now lets you use UPnP or manual Port Forwarding or whatever you need to do in order to get ARC working.
Now, if you do use FiOS it appears this router needs to be on the network but not plugged into the ONT. I’ve not tested this but the following diagram should get your TV back:
---> (wan) |G3100| (moca / lan) -> double NAT stuff / ONT -> (wan) |_your_ router| (lan) ---- \ ---> roon and other important stuff
Hope that helps. It may take a good reboot of all your stuff. Remember to power on from ONT out.