Apple Watch Ultra 2 won't download music – also watch wifi issue?

Recently purchased and set up an Apple Watch Ultra 2, now running watchOS 10.0.1 with an iPhone 14 Pro with iOS 17.0.1. When I attempt to download music to the watch while it's charging—either from the Music app in the watch or using the Watch app on the phone—it says that it's waiting for a faster internet connection. However, there is strong wifi at these times: speed tests on the phone indicate over 350 Mbps. A speed test app on the watch sometimes says in the range of 40 Mbps, but also sometimes as low as 2–4 Mbps.

The main problem is that the watch says it's waiting for a faster internet connection when the connection should be fine, and will not download a playlist while charging as it should; it just always says it's waiting. The related concern is that, according to the speed test app on the watch, the download speeds vary wildly, even while the nearby phone and other devices have no connection issues or such variability.

I have tried turning the watch's wifi off and on; restarting the watch; and turning off Private Address in the watch's wifi settings.