The units generally used their own proprietary signal from controller to drone and back.
Having flown my drones WAY out in the Gulf, there was no cell or wifi anywhere close.
Although you can fly the drone without a display, unless you're very familiar with your set up, I wouldn't recommend it.
The phone/tablet is for the display and the instrumentation.
If worse comes to worse, borrow a friend's phone and use that or find an old one with adequate horsepower and use the wifi to download updates and all that.
In the two photos, the one with the arrows shows the police cars and where I was standing during the drone photo shoot.
AD

