How to fix Apple Music offline playback issues on iPhone 11?

I have an iPhone 11 with latest version of iOS and have been having a re-occuring problem playing downloaded apple music offline without cell or wifi connection. I fly frequently and this has become quite annoying as the music will not play. I've done the recommendations from other discussions - Disabled Dolby Atmos, Disabled Lossless Audio, disabled sync library (although I was required to enable this again in order to download songs).

Downloaded music will play for a period if a internet connection is established and a downloaded song is started. This is a workaround for short periods of being offline but for longer periods the downloaded music eventually stops playing.

We have a apple music subscription with family share plan with 6 devices, the issue has also occurred on other devices.

Please help as we have some longer trips planned where we will be offline for extended periods and don't want to be without music.

[Re-Titled by Moderator]