With the P3 Pro, the controller connects by usb or lightning cable to your device.
In the field there's no need to have data or wifi, in fact you might turn it off if present as there could be some interference to the Phantom's own wifi link. 
You'll need to connect to home wifi or data to download maps which can be done in advance, also you need to be able to go onlie to register it initially and do any updates.
Not sure how the registering part works with a used one.