Welcome to the forum!
Your iPad does not need to have cellular. You can either cache the maps of your flying location beforehand or use your cell phone as a hotspot out in the field to connect to if you want to have the map view. If you don't want or need the map view then you don't need to cache or connect to your cell phone.
Just to be clear, you don't need cellular or WiFi to fly your drone. Not sure what P3 you have but if you have a Phantom 3 Standard the iPad WiFi just connects to the drones WiFi via the controller so that you can get telemetry and video through the camera. Most of the other P3 models use a direct cable connection (iPad to Controller) to get the telemetry and video called Lightbridge.
WiFi only is fine. You can open the Google Earth app at home (WiFi) right before heading out and call up the area you will be flying. Just don't completely shut down your tablet after doing so to keep that aerial imagery (map view) in memory for your flights that day. Then, while in the field, call up whatever flying app you wish to use and that imagery will appear on the screen. Some apps (like Litchi) will allow you to permanently save aerial imagery for future missions in the same area.
Geoff 1234, Welcome friend…1st thing….Get insurance, I don’t know where you live but in NC, State Farm insured all my drones( personal property policy) for $60.00/Yr
I’v had one claim 3 years ago and within a week, $1500.00 in the mailbox. They were very nice and I’m still with them. Also just take your time and be careful….Have fun……….