You will need to contact your ISP and ask if CG-NAT is being used. If so, then see if your ISP can provide a static public IP address for your connection. If this is possible, some ISPs charge for the static address, others may not.
One alternative is to use a VPN such as Tailscale. Some Roon users have had success in getting ARC to work over Tailscale, and example is here: