Phot slideshow won’t play My Music Library tune

18 months ago I set up a slideshow from my Phot Library. At that time I was able to play an appropriate tune from my Music Library on my phone. It’s an iPhone 6E.

since then I have regularly updated the IOS, which is now 17.6.1. However, I can no longer access the same tunes, only those purchased through Apple Music.

if I try to link the original tune by selecting Music Library, the icons for the tunes appear for a second, then disappear.

is these a simple solution to fix this problem? I would need a step by step guide as I am no techy, and quite old!

if it can’t be fixed, please say so, to avoid future frustration.

thanks