After numerous email and phone discussion with a variety of individuals, I finally learned the reason the WiFi didn't work because it was missing a critical component that was supposed to be delivered via an OTA update in the spring.
Well, I got the updated a month or so ago--and the hotspot still didn't work. More email and phone discussions--and I was told the "eSIM activator" would be needed, via a separate OTA update. Sigh...
As of last week, despite no notification of any sort of OTA update, I was able to activate my eSIM within the app and connect my car to my AT&T account--but, at $20/mo (and Mini's crappy customer service) I decided it wasn't worth the effort.
For those of you willing to pay, however, it is now apparently fully functional.