I have an Amplifi instant router in bridge mode. ARC works for me after I de- and re-activated UPNP on my main router (which acts just as modem and DCHP server, but no WiFi, I use the Amplifi for that).
Well, it took me over two hours to access the admin settings of the Xfinity router, but it looks like it may be fixed after enabling bridge mode. Finally accessed after unplugging the router and plugging it back in manually. My Harmony remote stopped working afterwards but “forgetting” the hub and rerunning it has fixed the issue. Xbox also required setting up the network again. ARC says it’s good through Roon settings. I’ll leave the house later and test it out. Thanks