Oh, I didn't catch that it was an iPhone. iOS is awful about this. A few years ago I bought all episodes of a TV series and a couple of films to watch on an upcoming plane trip, so I'd have a choice of things to watch. I made sure they were stored locally the day before I left. When I got in the air it had deleted them ALL to save space, even the ones that were in Google Play, even though I had plenty of space*. It was a work trip for an Apple-centric company so I had the iPad anyway and was trying to save on luggage by leaving my laptop. NEVER AGAIN.
Get a cheap Android tablet with a SD card slot, install iTunes long enough to convert all your tracks to MP3 and put them on the card, and celebrate your emancipation. I'll send you my old ASUS Zenpad 8 if you need it.
* The airline had a movie download service, so I got halfway through Fantastic Beasts, and then when I got to the hotel room to finish watching it their movie app wouldn't let me because it only worked on an actual plane.