Hello,
The upgrade range extender, allows you to access internet WHILE connected to phantom, mainly used in ground station situations, this way you an access google maps while you fly, to set waypoints and such! I'm not sure if it can/can't be reversed? But you can always turn off your phone data on your phone and only use wifi to not run up your bill, I believe it only works with IOS FORMAT ( Apple stuff ) I don't think it works for android? Hope this helps!
It's all in the user manual at DJI.com.
J Dot