Apple Watch won't play Apple Music on Beats Fit Pro

I've been trying to leave my iPhone 12 behind at home when I go on walks or to the gym. I have cellular service on my Apple Watch Series 8, so I figure it's one less thing for me to take with me. The problem is that when I try to listen to music with my new Beats Fit Pro - which work great with my iPhone - Apple Music just won't play when I'm away from my iPhone. I'm a subscriber to Apple Music. The Beats are paired, and if I try to play anything from Apple Music on my Apple Watch, I'll hear about 5-10 seconds of the song, and then it stops. Doesn't matter if it's downloaded or not. Thoughts?