Are you trying to use the KD8CEC software or just writing your own sketches for the Nextion.
In relationship to KD8CEC, Ian did some fancy footwork in his approach where he created buffers of commands/datas that he transferred back and forth between the Nextion and the Raduino. He did not use the standard Auduino library. On the Nextion side, this meant that he used timers and events (like a button press) to actually program that guy. BTW: Becareful of the Nextion code. Spaces matter! Especially in if/elseif statements.