The Service Center called me today to say that the parts for my car (new Pilot Screen, new computer, new computer cooling system) should arrive by January 20, at which time they'll transport my car in for repairs. They said they intend to keep it long enough to be sure all the problems are resolved, which is what I want. As I was updating the service manager on the issues I'm having, I mentioned what the Customer Care person told me about AT&T. He said that his understanding was that the cars use Verizon as the carrier, and he surmised that different cars might use different carriers. (T-Mobile 5G and Verizon are the two strongest of the weak signals I get at my house.)
End of the day, who knows?