Plan: Premium

Country: Australia

Device: iPhone 7+

Operating System: iOS 12.1.4 

I have several local files in a Spotify playlist that I would like to be able to play on my iPhone, but they're not syncing. I've gone through all of the instructions/solutions I can find on the support page and nothing seems to work; with my phone and computer on the same Wifi network (and with the desktop app open), I've tried putting the local files in their own playlist and downloading it on my phone and computer apps, but while I can see the playlist, the songs are invisible so obviously I can't download/play them.

The mp3 files are already on my phone in iTunes, I've ticked "show unavailable songs in playlists" in my settings, Spotify is listed as an exception to my Firewall, UPnP is enabled on my router network, and I haven't exceeded the 3333 download limit. I've restarted both my computer and my phone. Nothing seems to work - the songs don't even show up greyed out on my phone. Any ideas?