Did you make sure that all of the trims matched up between the primary TX and the buddy box? It sounded like when you hit the trainer switch, the plane veered to the left? On the ground, you need to flip back and forth on the trainer switch, and make sure that all surfaces stay put. Then you need to try it in the air a couple of times to make sure it behaves the same way.

Also, make sure that the buddy box power switch is off.

Good luck