How to sync local files with iPhone via 3G / 4G network?

[user-removed]

So i have added around 100 local files that arent avaible on Spotify, but im not able to sync them with my iPhone X (latest version of spotify). I have crated a specific playlist for the local songs, and the playlist is synced with my iPhone, but the entire playlist is empty when i enter it. **bleep** am i doing wrong?

I have a direct conenction from my computer to my broadband company and usually sync my stuff via 3G / 4G network.