Sharing this for others with the same problem.
After installing the NordVPN application on my Ubuntu 22.04 server with the Roon Server (Core) on it I found that my Android Roon Remote couldn’t find Roon Server anymore. Strangely my Roon Remote with Wine on my linux box had no problem.
My solution: setting option lan-discovery in NordVPN on the server to enabled.
nordvpn set lan-discovery enable
Cheers,
Joost