The device you are using with the Phantom needs to load the maps. Connect it to the Internet and launch the DJI app. Load the maps for the area you want and scroll around. Now disconnect and connect to the extender.
Using Googles Map app and the offline maps save function they last 30 days. However, from what I've read from other people they last less cached by the DJI app. I'm _guessing_ a few days.
You may want to do as mentioned above; Maps, go to the area and then save offline (warning, this is now a pain in the butt. Read online from Google on how to do this.). I'd then also cache them in the DJI app.