Hi @Ian_Hudson,
Thank you for reporting in on your progress, and I’m very sorry to hear that you’re joining the ARC users stranded by carrier-grade NAT without paying extra to your provider.
Note that Roon is diligently researching alternative solutions to port forwarding and additional workarounds in the meantime that will help democratize access to mobile playback to more users, regardless of the infrastructure of their service provider. Your posts here will assist in this effort, and I’ll add your service provider shortly to the list of known CG-NAT blockages here: ISPs and Routers: List of Known Solutions and Workarounds
In the meantime, please take a look at the unofficial, alternative solutions to port forwarding accumulating in the #tinkering and #roon section, like this: Tailscale implementation with ARC to circumnavigate ISP CGNAT