Sorry for the delay in getting back to you, things got a bit busy after release and we’re still working on catching up.
What is the exact error message that you get in the Roon Settings → ARC Tab? Can you copy and paste it here (without the IP address parts)?
Is your FritzBox in “modem only” mode? I suspect that you may have a double NAT situation here if it is. Fellow users have posted a guide for Fritzbox port forwarding here:
And there is a guide for port forwarding on Orbi here: