SOLVED
For anyone reading this, here is how it was solved after over 5 hours on phone/ chat to technical support, and several more hours actually doing the change. The problem was the software was glitching and needed to be completely re-downloaded from scratch.
• Plug your phone into your mac computer and do a full backup
• Turn off Find My iPhone (I had to go to Settings>Screen time> Content & privacy> turn On the Content & privacy restrictions to be able to turn off Find my iPhone)
• With phone plugged into mac computer, in Finder window (where you did the full backup) click the Restore iPhone button (up in the Software section of sync window). This will wipe your phone completely, and download new software
• After the 'hello' screen, when it asks if you want to set up from backup (or similar) select the option that says 'Don't transfer anything' (or something like that)
• When at the home screen when it's finished setting up again, do a Sync to make sure the music is now showing up and playable. Once you have confirmed this...
• With phone plugged into mac computer, select the 'Restore Backup' and it will reload your phone with all the previous info, it will look exactly like it did before being wiped, only difference is new software that plays ALL your music
