Phone apps will work just fine when GPS data is unavailable. You can either download the map before you go or just wait it out. The phone downloads more of the route than you see on the screen so you would have to be out of network for an extended period of time to lose tracking. If you are traveling any highway in the US you will pick a signal up again pretty quickly. Even traveling from Phoenix to Las Vegas on US60 where there is no Verizon service the maps keep going. I have used the Garmin products since the early 2000's and can attest that they are great products. They are just outdated in the capabilities compared to the phone mapping products due to the limited storage and always out of date mapping information.
