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: