Syncing/access on iPhone 6s Plus

I'm trying to access Apple Music on iPhone, but 'Sync Library' won't stay on.

It turn on briefly, but turns off again.

if i try and turn on sync in Music app, i just stare at 'Connecting your sync Library' (Probably related)

how can i fix this ?

iOS version is 15.1, so not up to date, but not that old either.

i can sync ok on Mac and that runs Catalina still. I've tested both over Mobile Carrier and Wi-FI. To no solution