I use Mint Mobile here which is a MVNO that uses T-Mobile. I don't get cell reception at my house. But my sister uses AT&T and gets full bars here while my phone is dead. And by dead I mean it doesn't even connect.

So maybe went Gig went cheap when choosing a cell partner.

Having a good enough signal to make a call is not the same as having good enough signal to push data, and if the car expected to move a megabyte of json over LTE to get started, it would explain the behavior just fine.