Another Roon user used port forwarding on their BGW-320 router to correctly route ARC traffic, the instructions to do so are linked here:
I haven’t seen the IPv6 error message you have listed (AT&T uses both v4 and v6 dual addressing schemes so that one is a bit of a surprise), but the port forwarding video may be able to resolve your ARC issue. I have an older 5268 gateway with AT&T and it’s interface is different than the BGW series.