My downloaded Apple Music songs keep disappearing on iPhone

I travel internationally and download playlists as I don’t want to burn through roaming data listening to music.

frequently when I go to listen some or all of the downloaded songs are no longer downloaded. How do I prevent them from disappearing. What’s the point of paying a subscription if you can’t get access to the music you’ve paid for?

[Re-Titled by Moderator]

Original Title: My downloaded songs keep disappearing