First thing to look at... Have you tried using a different OTG cable between the controller and the phone?
2nd thing: are you constantly pointing the control antenna towards where the drone is in the sky?
3rd: are there any trees or solid objects blocking clear line of sight between controller and drone?
4th: in the menu screen, tap the transmission tab and check the state of the control signal... if there are loads of red spikes: tap the box marked 'DUAL' (top right corner) and manually select 5.8gH.
5th: don't fly further than Line Of Sight... it might be that your signal is weakened by distance.