I am able to play multi-room (synced, playing same track) with multiple Mu-So devices but I cannot include my Uniti Atom in the multi-room. It doesn’t even show in the menu as a multi-room option if I use one of the Mu-Sos as the master. All are connected to the same network with the Mu-Sos accessing it wirelessly and the Atom using a wired connection. Multi-room worked across all of them without a hitch until about a week ago.
Rebooted the network, power cycled the units, re-installed Naim app. None of that worked.
I switched over to Chromecast for multi-room and that did work. Then I switched back to native Naim multi-room and it somehow had fixed it and all of the units were available to me.
I have a vague memory that for some files you can’t multiroom if the muso is the master, but it would be ok with the Atom as master. It may be that I am misremembering that and it might also be that I am recalling an old firmware issue that has been fixed.
Anyway if multiroom doesn’t work, try with a test file from a source that you do know does work.