macOS 27 Golden Gate restricts Roon’s network access at the OS level because it falsely flags the Roon application bundle as a threat.
The team is working on a solution to restore device discovery on Mac-hosted RoonServer machines. We’ll announce this broadly on Roon Community as soon as it’s finalized.
In the meantime, there’s an unfortunately circuitous (but effective) workaround you’ll need to take, outlined at the link@Suedkiez shared above.
This requires a few Terminal commands to reestablish Roon’s connection with the loopback ethernet interface. Since it’s deep at the OS level, you can’t take these steps within system settings or within Roon alone.