@Andrew_Crossley, welcome to the community. I moved your post ot the Port Forwarding support category. To better assist fellow users such as me and the Roon staff with your question, can you please paste the error message you are receiving in the Settings → Roon ARC tab on your Core?
Thank you Andrew. Unfortunately, the second IP address shown, starting with 100.64, shows that your ISP is using something called Carrier-Grade NAT (CG-NAT) which is required to share a small number of public IP addresses across many homes. Unfortunately, Roon ARC cannot work at the present time across networks using CG-NAT. They are looking at options to make this possible, but have not publicly announced a timeframe or a solution.
There is a possible option if your ISP can remove CG-NAT to your home or offer a static public IP address. Is this something you can discuss with your ISP?
Altnernately, there is another option to use a VPN service such as Tailscale, but I do not use that type of service for ARC. There is a good discussion on using Roon ARC with Tailscale here: