According to your images your UPnP server is SqueezeNAS . That puzzles me as I can only find a related reference to Squeeze Neural Architecture Searching. Also where it is shown at the bottom of the screen would normally display a UPnP icon. As you seem to be aware the correct operation of Mosaic is dependent upon the UPnP server being MinimServer for the various reasons that you can find in the FAQs.
Your images are also not of the screen that I was expecting as they show information at track not album level which would normally be the screen that appears when the album is actually playing rather than the screen for selection which is where the Play All button appears . However it is also clear from the right hand part of the display that looks like a phone or DAP that nothing is actually playing.
So back to basics. Close Mosaic. Now reopen it and once it has found your device ( Rossini Player) you should then see a screen with a list of options starting with UPnP, then USB, Deezer, Qobuz, Tidal, Radio, Podcasts, Spotify.
Select UPnP and you should see your server listed. Select this and you should have a screen indicating the number of albums and items ( tracks) on your server. Select the number of albums then Show All . You should now see a screen that has the Play All button and a selection including View All. Select View All and you should see a screen with the Play All button and your albums listed below.
Now the exact way all of this appears will reflect the “dialogue” between your server software and Mosaic. As you do not have MinimServer it may not appear as I have set out. Please tell me if and how it differs.
Which server do you use? Can you install MinimServer on it?