Marin county is very, very hilly. Somewhere between hilly and mountainous, given that Mt Tamalpais sits smack in the middle of it. While most of the population is close to the freeway on the inland side, there are many many lightly populated areas with awful or no cell reception and issues with high-bandwidth type connections of various sorts. Many of the ocean-side areas are largely cut off from the main population by the mountains. I've never been to Dillon Beach, but it doesn't surprise me at all that Marin is where something like this would happen!