[greglepore]

The issue is the one raised by the op. If you don't have connectivity when you start the route, a phone based gps won't calculate a route. It'll know where you are but lacks the ability to route from there.
We have this issue at home. We have no cell service here or for a few miles in each direction. We also have a cabin on the property we abnb. We have to warn guests that if they need gps to get where they're going to make sure they start the route before they leave our guest wifi (luckily pretty wide range mesh) or their route won't load.