There are giant swaths of area with no coverage, especially in the mountains of arizona, including the freeways and especially highways. The entire western US can be spotty with signal out in the great wide open. It isn't until the Midwest and more east that one should largely not worry about signal coverage anymore.
Arizona toddler rescued after getting trapped in a Tesla with a dead battery | The Model Y’s 12-volt battery, which powers things like the doors and windows, died ( www.theverge.com )