I hope this post saves others from wasting a couple hours of their lives like I just did.
Like other ConnectedDrive-capable BMW buyers in the US, I got a document from BMW saying "you MUST REGISTER [your Signal Booster, also known as an LTE Compensator] with your wireless provider". For my wireless provider, AT&T, the URL
https://securec45.securewebsession.c...albooster.com/ was provided.
First, that link is a dead end. It leads to "Not Found [CFN #0005]". The attsignalbooster.com site that used to exist is likewise dead.
Second, don't bother calling AT&T. If you speak to anyone in the "Connected Car" department (I've spoken with 3 customer support agents in that department, and 3 others in different AT&T support departments), they will sound like they have never heard of a signal booster, and will eventually tell you to contact BMW. BMW will respond by telling you to contact AT&T...
So it seems impossible to register your signal booster with AT&T.
Thus, using the charging pad in your new BMW while using AT&T's cellular service puts you in violation of US FCC regulations.
Don't worry. No one at the FCC or at AT&T seems to care about this outdated 2014 regulation.
Most importantly, your signal booster—which works only on a phone that is on the charging pad—will work whether or not it is registered.