Big plot hole no one is talking.[Spoilers]

There is no way there would not have had a mobile signal up there. They said the radio tower was disused and there was just the red light there to warn aircraft. It was in a bad state of disrepair so probably scheduled for demolition at any point.

Radio waves travel best when there is nothing blocking the signal. They were at the best place to have a signal! The whole thing about there being a signal at the bottom of the tower, and clearly a signal at the diner/hotel would mean they had a signal there. There is no reason why they wouldn't have done. But I guess movie has to movie. They did say about there being interference. Well, clearly there wasn't as the drone wouldn't have worked either.

Also iPhone batteries do not last that long!