I have always flown my P4s with an Iphone and put it in airplane mode only.  Never had any issues with RTH if needed.  I was given a deal on an Ipad Pro - wifi only.   Then was told that I can't use RTH anymore.   I had thought it still worked completely normal when flying, at least in P mode.   Can someone clarify this for me please?  I have even flown in areas where there is no cell phone signal at all, but out of habit had always put my cell phone in airplane mode.  Want to get a better understanding of this if I have been wrong.    I fly 2 Phantom 4s and should be getting a P4P soon.

Then was told that I can't use RTH anymore.

You have been told this by an idiot.
The Phantom has a perfectly good GPS that it uses for all flight functions (including RTH).
The only things you can't do with a non-GPS iPad are:
1.  Follow-me flight (but you can do active track)
2.  Reset home to the current location of the controller (which you might use on a moving car or boat)

btw  Your Phantom doesn't need cell service at all and you can switch to airplane mode if you like - or not.
It won't make any difference.

Agree with Labroides,

Chinook, you can check this function through user manual. Please check more detail from the attachment.

Thank you so much both of you.  This person is a self proclaimed expert and by the end of the conversation had me doubting myself.  I knew I couldn't use follow me, but figured if I ever do use that, my cell phone is always there. Wanted a larger screen for the most part.