Local File refuses to sync to phone, disappears from desktop

[HeySamKermode]

Plan

Premium

Country

Canada

Device

Samsung Galaxy S8, Asus Laptop

Operating System

Android Oreo, Windows 10

My Question or Issue

I'm trying to add local files to my library to listen on my phone, and I've gone through all the steps on the local files help page. My file is in mp3 format. I tried downloading quicktime, a codec. I've done everything I could think of. No matter what I do, nothing works. The file plays fine on desktop. I add it to a playlist (or make a new one). I download that playlist on desktop. I open spotify on my phone and the track is greyed out and won't download. As soon as I navigate away from the playlist on spotify desktop, the track disappears. I don't know what to do anymore, I've browsed through what feels like hundreds of forums online and have had no luck resolving this. I'm at my wits end.