Anyone know how to stop my iPhone music app from undownloading songs on my phone? ?

I recently turned off cellular data for my music app on my iPhone 8 because it was using up way too much data, and now I’ve noticed some of the songs don’t stay downloaded on my phone for more than a few hours. Like, they’ll be downloaded, then a couple of days later they’ll be off my phone and only in the cloud, so I can’t listen to them when I’m out unless I turn on cellular data, which I don’t want to do. Is there a way to fix that?