Music stops playing in low signal areas

I use a usb connection to play music in my car. I have all my music saved on my iPhone and do not subscribe to Apple Music I also have it disabled and web streaming disabled in settings. Whenever I go through a low signal area the music stops playing and carries on when the signal comes back, this is very annoying as I live in the countryside. If I put my phone in airport mode then this problem disappears so clearly it’s the signal dropping that’s stopping the music playing but why? If I leave it in airplane mode permanently then I have no phone access. Also I have the same problem in my wife’s car, so clearly it’s the iPhone