Welcome to the community David.
I’m just a Roon user like yourself.
You may want to try toggling the UPnP setting on your router, as well as reboot the router. For more information, you may also wan to review the Knowledge Base Port Forwarding Troubleshooting article. More specifically the section “Generic Error (unknown/undefined)”.n
If that does not help, I also found this post specifically about having Spectrum as an ISP, although I’m not sure if it is still relevant as I believe that ARC has been adapted for IPv6 compatibility.