The switch selecting the temperature is the thermostat. The function switch only selects which elements to turn on. Not getting up to set temperature would nearly always be the thermostat. Who knows what is happening without being there to test & see. You don't appear to have any test equipment or fault finding skills. I suggest you call in an expert, otherwise you could just go around in circles and spending money on unnecessary parts replacement.
Lincoln Appliance NZ, November 2017