Hi @leftside,
Please accept my apologies that you’ve been waiting for so long for a staff response to your issue.
Looking at the screenshot you shared of the UPnP port mapping assignments from your router, it’s very likely the issue you reported was in fact a bug that we’ve since pinned down and resolved. The most recent release of both Roon 2.0 and ARC (released today) contains a fix for this issue which might resolve your problem and allow for port forwarding. Please see the Release Notes here.
Port assignments were defaulting to the wrong range in certain rare cases, causing port forwarding to fail both when automatically or manually configured correctly. I suspect if the port range Roon has chosen starts with 24 in your case, that it was certainly a range reserved by your ISP and not available for port forwarding.
I recommend updating to the latest version of Roon 2.0 and ARC on all devices in your network, and then attempting to configure this rule again with the updated port assignment listed in Roon → Settings → ARC. Let us know if that doesn’t help.
We’ll be standing by for your response, and thank you again for your patience.