Check to see if you can download more territory.

On my car, deep in the options, is a screen that shows CONUS, and lets you draw a box around the portion you want to download.

I have a box drawn around an area about twice the size of California. Hopefully your car has enough storage for that size, too.

On those occasions when I'm out of a cellular service area, the map shows a banner reading something like "No cell connection. Using downloaded maps."