Can't sync music from remote library

I'm running Catalina and trying to sync a playlist from my MBP to my iPhone. I Don't subscribe to Apple Music.

My music is *only* stored on my Synology NAS (I don't have 'Copy files to Music Media folder' ticked because I don't want to use any space on the MBP. Apple Music picks this up fine when I specify the NAS as the media folder location.

However when I connect my phone, select the playlist I want and click apply in Finder, it shows it syncing (as well as showing it syncing in Music), almost immediately spends about 15 seconds on step 2, 'Copying diagnostic information' and then finishes without error, but nothing is synced to the phone.

Does Music not allow syncing unless the library is on the local disk or am I missing something?

Thanks in advance for any assistance.