@Philip_Gibson I have a Plusnet Hub one router and it’s config screen is the same as yours. After much head scratching I’ve managed to sort it so I think it will work for you too.
Once you’ve created your ‘Game/Application’ with 55002, you then need to go a step further:
Go back to Port Forwarding and select the ‘Roon Game/Application’ from the first drop down. Then in the second drop down scroll all the way to the bottom and there is the option for ‘User defined IP address’, click that and then input whatever the IP is from your Roon ARC page.
Hit apply, and then go back to the Roon ARC page and refresh - for me it immediately said it was Ready. I downloaded the ARC app and all is working as advertised.
Thanks for your suggestion, @Red_R. I’ve followed the steps you’ve outlined and I’m still getting the same error message. I’ve also changed the settings on my Asus router, as per @Bob_Scratchit’s suggestion. Frustrating!
To isolate the problem it would be useful to contact your ISP to find out if you are getting a public ip or a cgnat address.
Cgnat does not allow port forwarding which Roon ARC needs to work. In this case you may be able to get a real IPv4 address from your ISP.
Likewise, no 2 routers with activated NAT should be connected in series in your network. The first router should be in bridge mode or add a port forwarding rule to the second router to avoid a double NAT layer, since ARC port forwarding does not work with it.
Carrier-grade NAT usually prevents the ISP customers from using port forwarding, because the network address translation (NAT) is usually implemented by mapping ports of the NAT devices in the network to other ports in the external interface. This is done so the router will be able to map the responses to the correct device; in carrier-grade NAT networks, even though the router at the consumer end might be configured for port forwarding, the “master router” of the ISP, which runs the CGN, will block this port forwarding because the actual port would not be the port configured by the consumer.