In a standard Tailscale ARC setup, you’d install Tailscale directly on your RoonServer machine and on your phone.
STEP ONE
Install Tailscale on iOS/Android
On the phone you use for ARC, download the Tailscale app and install it.
iOS: https://tailscale.com/download/ios
Android: https://tailscale.com/download/android
STEP TWO
Log into your Tailscale account in the RoonOS Web Administration Interface or Nucleus Web Administration Interface.
STEP THREE
Test your Tailscale Subnet Router and connect RoonServer to ARC.
Navigate to the RoonOS Web Administration Interface and verify that Tailscale has logged in.
Disable the port forward in Roon → Settings → ARC by entering “0” in the port value. If 0 is not available, try entering any integer that causes a “Not Ready” status
Next, turn on cellular data only (disable WiFi) and test the Tailscale connection.
Community Resources:
As of this writing, the best source of knowledge on Community for subnet routing in Tailscale lives here: https://community.roonlabs.com/t/tailscale-implementation-on-rock-nuc-to-circumnavigate-isp-cgnat-for-arc/215356