I installed the Roon Core on my Synology DS720+, then I tried to connect with the android app installed on my pixel4a over mobile data connection.
The Roon app is not connecting. It says it needs to be connected to the same Wi-Fi as the Roon Core.
Really? Can I set somewhere my hostname to connect remotely?
No, Roon is not built to use any VPN and does not support VPN usage. Everything has to be on the same local network, whether Wifi or Wired, and on the same sub-net.