Hi @Urs_Stirnimann,
The giveaway is in this line: ConflictInMappingEntry
The port you are attempting to use to route Roon from your Core to ARC on your phone is already mapped to another service; this is the result of other software or hardware on your network or, more likely, the result of your service provider’s carrier-grade network address translation. As @Rugby mentioned, you’ll need to inquire if Nokia can provide your 5G unit with a dedicated IP address.
Please see this thread from another Swisscom user with a 5G setup: ARC through SIM mobile receiver - #3 by Steve_Udriot1
The Roon team is working on long-term solutions that should enable ARC to work out-of-the-box for more users with CG-NAT from their service providers. In the meantime, please try contacting