You do not need to insert a SIM and/or activate the cell service in order to use the GPS receiver on an Apple device. While only GPS is required to use Follow Me, you'll need an Internet connection in order to download the maps. You could
cache the maps in DJI GO in a location where you have an Internet connection before you head out to fly or tether your iPad
Mini 4 to a device that is sharing its Internet connection via a hotspot.