Song Not Downloaded message on iOS but can play on PC

[thinkOfaNumber]

Plan

Family Premium

Country

Australia

Device

iPhone 6s, Dell Laptop

Operating System

iOS 12.1.2; Windows 10 pro 

My Question or Issue

A lot of songs (90% of a 200 song playlist) are greyed out on my phone. I know that sometimes an artist or label removes songs but this is not my issue as they're available on my PC. Here's how they look on the phone, you can see the top one is white and the rest grey:

If I try and play the song on the phone, I get this message "Song Not Downloaded. If you own the song, you can sync it from your computer". I haven't imported any owned songs - these are all played from my subscription.

"Learn More" tells me some steps about syncing:

I've tried to follow these, but there is no "Offline Sync" button on my computer nor on my phone, much like another user found [can't post the link, will try later]

• If I use the "downloaded" toggle on the phone it only downloads the non-greyed songs
• If I use the "downloaded" toggle on the PC, almost all songs download to the PC but nothing changes on the phone
• If I play the songs on the PC, my phone shows me that I'm playong on another device, and the greyed song in the playlist shows that it's playing

I have tried deleting the phone from offline devices. It never comes back, although I can still stream and play downloaded songs on it.

I have also tried deleting spotify from the phone and all it's data, and reinstalling & logging in - no change.

Can anyone help?!

[thinkOfaNumber]

Any ideas whatsoever, from anyone? I'm paying for something I can't use...