The Roon app on my MacBook is unable to connect to Roon Core on iMac. However, my MacMini can connect to the Core. The MacMini is connected to the network via ethernet and the MacBook via wifi.
Had a rethink about my set-up and consequently I Installed Roon Server on the MacMini and Roon apps on the MacBook and iMac (and iPhone and iPad) and everything works well now.