Thanks to you all so far!
Both my ISP for fibre net at home and my mobile provider have fully implemented IPv6 and I see both my IP6 when using one of the IP checkers.
Any attempt to use IPv4 or opening ports for IPv4 has failed as my ISP uses some kind of CGNAT resulting in a “MultipleNAT” error in Roon regardless the port forwarding method. They told me that they do not deactivate this nor offer any IPv4 option in my contract (I do not want to upgrade to a business tariff).
Switching to Roon build 1257 early access as well as ARC beta and opening the port forwarding to IPv6-only seems to solve the problem as Roon is now showing as a result of the ARC test procedure “Your Core has been successfully configured and has confirmed to be securely accessible by Roon ARC”.
When running Roon ARC on my phone being connected to the very same network everything works. But from mobile network ARC app is showing notifications that it cannot reach the Roon core at all and that it is not properly configured for reaching it externally. The port seems to be visible and open to the Roon ARC check. If I do a port check from outside using my IPv6 of the Roon core and the port shown in ARC menu I get a reply that it is open as well.
I assume that some kind of firewall might be the reason. But I have no idea where. In my router’s menu I have opened the firewall for this port and my server seem not to offer any possibility to do that manually.
Any idea where to search for such option?