In the interview it indicated that at one point she did have enough cell service to ask her phone for directions. It couldn't tell her a route but could tell her she was ~20 miles from a Starbucks (that Starbucks was on eastern side of the Sierra Nevada mountains meaning she would have to cross over drainage from where she was to other side of the mountains).
So I'm not sure how much of an issue no cell service played as much as that she had traveled on roads the online map knew to be closed. (Almost the opposite issue where sometimes travelers got into trouble following GPS instructions on roads closed for the season - hence there is a reason for GPS to not route people along closed roads).
I use downloaded maps, with GPS, with the cell service turned off. Works well. It shows my position on the map, and I can navigate, manually, from there.