You were in the right place to add the thermostat, but sometimes the thermostats have to factory defaulted to pair, yes even when they are brand new. To do that, press and hold the black reset button and the hold button at the same time, let go of the reset button while still holding the hold button, when the temperature returns to the screen let go of the hold button.
Factory default the thermostat while the touchscreen trying to fin it and it should discover it. It should find and pair itself.
