I don't think the phone is an issue. I used Chinese dual sim phones for years with no problems. Since I switched to Three I've been using a Samsung Galaxy Note 4, a cast off from the older not so young lad. Hard to imagine the Chinese phone would be superior to the Samsung... in fact the main differences are that the Samsung has a better camera and better WIFI reception. There's no difference in phone signal reception or mobile data reception. It could, of course, be simply down to the mytaxi app/servers deteriorating, perhaps on account of increased traffic.
Last night, for example, I had to force stop the app when it lost connection for a prolonged period during a €40 account job. I couldn't clear the job at the end of the fare as I had no connection so had to park up and fuck about (force stop) with the app after the customer was gone. Of course, that opens up the possibility of the rider cancelling the fare before it's processed... didn't happen on this occasion but it could happen!