I think the root cause for Deutsche Glasfaser (FTTH), Cable or ADSL/VDSL are all the same.
It depends on whether or not you have an IPv4 address.
If yes, ARC works, if not, then ARC will not work.
Check (in your Fritz!Box) under Internet/Online Monitor, here you see whether you have an IPv4 address or not (DS-Lite-Tunnel).
See picture here for DS-Lite connection or here if you have an IPv4 address assigned.