Offline playlists become disabled after a few hours offline

[codermyers]

I have Spotify Premium installed on an old cell phone that has no data plan, only wifi.  I download offline playlists from my home over wifi, then use the phone as a music player while skiing.  

However, I have noticed the following recurring behavior:

3 or 4 hours after turning off wifi and leaving my house, the phone stops playing music and no playlists are available.  So I have music for the first 2 hours of skiing, then nothing until I get home and reconnect to wifi.  (At this point I have to exit Spotify and listen a few albums that I have laboriously ripped and sideloaded onto the phone with a different app.)

Any way to fix this?