The phone doesn't need to broadcast anything to control the drone directly. The phone talks to the remote control unit, which is what broadcasts signals to control the drone. You don't need wifi or mobile internet, or even bluetooth to fly a DJI drone (the phone connects by cable to the remote control unit).
(Actually, that's not 100% true -- if you're in a locked zone that requires permission to fly (such as near airfields or other protected sites), you will need internet access to start your flight and unlock the zone using your DJI account. Otherwise the drone may refuse to fly into restricted zones.)
You don't even need the phone at all -- the remote unit is quite capable of controlling the drone in flight with the phone switched off.
