carplay won't play music from my apple library unless I'm in wifi range

I have an iphone 11 with cellular service (no data plan) and recently connected to my new car using apple carplay. I'm trying to figure out why it will play music from my apple library while I'm parked in my driveway but not when I'm away? When I'm away from home Siri says my library in unavailable because I don't have an internet connection. I've tried uninstalling carplay and reinstalling it as well as switching bluetooth on and off. I've also tried with a wired connection and a wireless one. I don't have an apple music subscription. Am I doing something wrong??