Currently experiencing the subject of my post. Whenever i select a song (offline/downloaded) it won't play unless i have a wifi connection or my mobile data is turned on. badly need your help. thank you!
First try to use Remove Offline Devices and Sign Out Everywhere from your Spotify account page. The first tool will disable the download option but simply switch it back on so the songs become available.
If that won't help, go to Spotify Settings > Playback > Hide Unplayable Songs and remove them from the playlist(s) you want to download to see if that helps.