Upgraded to iPhone 15 & lost half my music - help!

I've always had issues with music on my Apple devices since iTunes was effectively replaced with Apple Music. I have lots of downloaded music bought years ago plus some more recent purchases but since upgrading from my iPhone 12 to a 15, I've lost lots of my music. If I go to a playlist, half now come up with an error saying, "This song is not currently available in your country or region." and they're greyed out.

I bought these songs and they still work fine on my iMac using Apple Music but not my new iPhone.

When I upgraded, I backed everything up to iCloud the restored to my new iPhone.

As the music is showing, but greyed out, I'm assuming it's there. Can anyone help please?