As far as I know you can’t download updates with premium connectivity, only by WiFi. Although you can use hotspot from a mobile phone to download when you get notice of an updateI am off (to gym). But see Post#37 and the following replies in this thread: Anyone Else.... It may help (or not).
To EVERYONE: Make sure you have a good, reliable connection to the Internet (e.g., strong connection to your home network) wherever the car is parked. I still have the premium connectivity thing, so maybe I get additional cellular phone Internet service in the car too? (Do I have that right? It seems like my Model X can download an update anywhere I am, around town.)