Downloaded songs won't play

[AneRanda]

I have this problem only on my iPhone, Spotify works perfectly on my computer.
I have premium, but when I download music - both albums and playlists - very often the songs won't play in offline mode. They appear to be downloaded, but they don't start playing until I turn on 4G. This is immensely frustrating, as this is the reason I pay for premium monthly.
I have tried everything. Reinstalling the app, logging off and on, un-downloading and re-downloading the music, but nothing works. So now I'm considering stopping my premium subscription.. Please help me with this..