Was wondering if you had to have cell service to fly the phantom 3 advanced? I was wanting to get an iPad mini but didn't want to put it on my plan? Can anyone help?

No you do not, the only thing you cannot do with cellular (iPad WiFi has no GPS also) is follow me as the device needs a GPS.
Also, without cellular you will not have live map data but you can easily cache the map of your flying area while you are at home on your WiFi. Or you can just use your phone as a hot spot so your iPad will have WiFi to get the map data. But that does not work with follow me, but if you must use follow me, then just use your phone anyways as it's much smaller and easier to manage while your walking around having the aircraft follow you.

You do not want any form of internet while flying. Turn your device to airplane mode. If you need the map feature, cache the map in your device just before flying.

Thank you!