I have always used my wife's Android mobile and google maps for the navigation. WHen we enter the route, the map application creates the route accessing the internet and once the route have been created, then it does not requires the internet. Only GPS is required. We have used it in the places where network was zero but still application was showing the route properly.
Unless you have offline saved google maps you can not have navigation work! Your device needs to download the maps as you navigate to new location. May be it would have worked for you for a already loaded map, but if you say travel 50km(just my approximation) then if you dont have net connectivity it would fail.
Please correct me if wrong. I never used offline google maps but that's the way it would have worked.