I’m new to all this so if my comment is rudimentary please forgive me! I just purchased an Air2S Combo and a Smart Controller.
I absolutely could not get the Smart Controller to pair with the aircraft. So I took the supplied controller, mounted my iPhone, started the app, followed the instructions, there were some updates and was able to pair the aircraft to the supplied controller.
After this I shut every thing down and started over again with the Smart Controller and it paired! It seems it needed the original controller and the app to sync everything before the Smart Controller would pair with the aircraft.
I hope this helps someone as this solution was found accidentally out of frustration.