Hi @Vince_Galet and all,
Unfortunately, it’s true that the particular IPv4 tunneling implementation used by T-Mobile 5G residential tier accounts doesn’t preserve port assignments, so returns from your mobile device using ARC are filtered out and unable to reach the Core. This will manifest as the frustrating experience you’ve been facing for these last months: endless timeouts in Roon → Settings → ARC, and a “Can’t Find Your Core” screen in ARC outside your local network.
This particular situation is further complicated by T-Mobile’s Arcadyan gateways, which do not support UPnP and do not allow for manual port assignment.
In short, Roon users with your internet setup are stuck without ARC outside the home for the time being. From the team side, we have an urgent and high-priority initiative to allow ARC to function with IPv6 addresses, among other tickets to ease up the obstacles presented by common CG-NAT. We anticipate this will solve many major ISP-level blockages with ARC or otherwise open up supported workarounds. The team will be posting notes in our general tracking thread for ISPs and carrier-grade NAT issues here: ISPs and Routers: List of Known Solutions and Workarounds
Download local content in offline mode will function, and while many VPNs will require installing software on your RoonServer device, it’s always worth exploring some of the #tinkering discussions like this one: Mobile Use: Roon 1.8/2.0 via VPN vs. Roon Arc