On your Mac machine running Roon Server, go to the Tailscale website and download the DMG installer for MacOS.
iOS: https://tailscale.com/download/ios
Android: https://tailscale.com/download/android
Restart RoonServer and ARC on your MacOS machine and your phone.
Turn off WIFi, leaving only the cellular data connection. Open the settings page in ARC and look for “Connected” to see if ARC is working.
If not connected, go into Roon and disable port forwarding. Go to → Settings → Roon ARC and enter “0” in the box for “Port”. If successfully disabled, you should see “Not Ready”
Some networks have port ranges reserved. If 0 or another number is not accepted as input in the “Port” box, try a few different numbers until the “Not Ready” status displays.

If you’re still having difficulty using ARC with Tailscale, please post on Roon Community in the #tinkering category, where users are accumulating Tailscale/ARC expertise.
We recommend engaging Tailscale support directly - they offer robust and responsive support and are better equipped to troubleshoot within the Tailscale app itself than Roon support. Please see their Support page here to reach their team.