Thank you for your post and I’m sorry to hear that ARC autoconfiguration hasn’t worked in your case.
First off, please see @Rugby’s post above. You’ll want to verify that you have a dedicated external IP address for your account.
The diagnostic message visible in your screenshot is a miscellaneous UPnP error. Unfortunately, it doesn’t give us a precise indication of what’s wrong with your network. On a positive note, the port test is quite sensitive to multiple NAT scenarios that might suggest an ongoing issue with your provider, and there’s not necessarily an indication of that here, either.
A few things you can try in the meantime:
Toggle UPnP on/off in your router settings
Verify your Asus firmware is up to date in the router. Sometimes, outdated firmware will trigger the router to disable UPnP processes for security precautions.
This third fix is less intuitive but has solved many related UPnP problems for users with your router. In the Asus router web GUI, navigate to the LAN IPTV settings, and make sure IGMP Proxy and multicast forwarding are enabled.
In that case, please see the suggestion above about clarifying with your provider whether or not your account is still subject to carrier-grade network address translation (CG-NAT).
They may have sent you the address but did you verify that it works?
You could go to your router settings and set up a manual port forwarding rule. In the rule, use the Roon Core IP and the Port 55002 that Roon shows you in the settings page under the ARC error. (And the rule will let you choose TCP or UDP. Use TCP)
Closed probably indicates it can be reached though it responds that the port is closed. I wonder if your port forwarding rule works.
If it were blocked, it would point more to an ISP issue. Though could be the router as well - I had a cheap ISP router and it showed the port as blocked until I installed my own better router.
In any case, this probably is the area to focus on
