I’m not sure what hardware you’re using, but I had a Rambo board with the external display. The display connects to the controller with a small, maybe 10 pin, cable. For whatever reason, the cable needed to be turned 180 degrees, reversing/inverting the connections. The cable even needed to be modified to cut the key notch off so it would fit.
Not sure if this is your issue, but this is a place to start.