Apple TV+ Videos stop after a few seconds

We have Apple TV+ running through a Firestick plugged into our TV. All the other apps and channels work fine. Trying to play videos on Apple TV+ we get an error message to recheck the internet connection, and the video stops. We tried resetting the account (read: Hassle), but that didn't work. We unplugged the Firestick and plugged it in again, reset the internet connection (we're using Spectrum Cable service), nothing has helped. Ironically, we can watch videos on our iPhone 13 with no problem over WiFi. What can we try next?