Note: Many of those areas without much phone coverage also don't have a dense road network, so turns are quite far apart and not uncommon that you'll follow the same highway all day.
This. I’ve done long days out west, including days on ACA routes, with few turns. A good example is leaving the Missoula KOA heading east. From the campground office you make a left at the end of the street and the first right. If you take the all-paved route you can make it to Lost Trail Pass without any more turns. The unpaved alternative adds two turns. That’s about 100 miles. Left at the top of Lost Trail and a right when you get to Wisdom and you’ll find yourself in Dillon without any more turns. Another long stretch. When you are in areas like that, make mental or written notes the night before and stow the phone until really needed. Even in small towns you can usually find someone to ask “Where’s the city park for camping?”
As for waterproof cases, I have been using the same LIFEPROOF case for my iPhone since 2017. Worth the money. And it comes in many colors to match your panniers.