Hi @Scott_Savold,
Unfortunately, T-Mobile 5G is one of the few ISPs known to interfere with ARCs out-of-the-box configuration due to their specific implementation of IPv6 and carrier-grade network address translation.
The Roon team is making progress on solutions that will enable port forwarding to function for T-Mobile hotspot users either via autoconfiguration (UPnP) or manual port assignment. We’ll be reporting progress in the main T-Mobile thread here: T-Mobile (US) 5G Home Internet: CG-NAT Blocks ARC, Workarounds Required [Ticket In]
Until then, I recommend exploring some of the more simple VPN solutions our users have accumulated, like this one: Roon Arc + Tailscale with T-Mobile Home Internet. Here’s another: Success with Tailscale VPN