You probably had success with ZeroTier as Tailscale currently does not support for broadcast or multicast due to the point-to-point nature of the connections. The problem is that Roon endpoint discovery is based on IP multicast.
Many have had success with Wiregaurd or ZeroTier and using the Roon app while in a different location.
OpenVPN enables the Roon application on iPhone to act as a controller for the RPi or any other playback device attached to Roon. I have not tried to use the iPhone as a music playback device.
I was having trouble with iPhone endpoint with ZeroTier - until today I enabled the option “Ethernet Bridging” for each device on the my.zerotier config webpage