iPhone - Music synced from iTunes on Windows (for over a decade) now "...not available in your country or region"

Many years ago I migrated my purchased CD collection into iTunes on Windows, and synced a selection to my iPhone. This has worked for over 10 years.

Since upgrading to iOS 26 I can no longer play anything on my phone.

None of the music tracks show up in Artists, Albums or Songs. Those tabs are completely empty.

In the Playlists tab I still see my playlists, and all the tracks are grayed out. Attempting to play one results in "This item is not available in your country or region".

To be clear, I don't have ANY music purchased through Apple Music, all of it is from CDs I have owned, in some cases, for 30 years.

Some data points:

• When I connect my phone to iTunes on my PC via USB, under "On my device" it still lists all the synced music.
• I can play all the tracks in iTunes on my PC with no issue.
• I can do a sync to the iPhone in iTunes and it completes with no error.
• I can play all the tracks on the iPhone if I'm on the same WiFi network, via Home Sharing.

It's only the downloaded tracks that can't be played standalone.

How do I fix this?