Make sure that the thermostat is properly powered and check that the HomeKit code used is the one displayed on the thermostat (see ;How to recover my Smarther AC with Netatmo connected thermostat HomeKit code? ;). If this is the case, reboot the thermostat by doing a 5 seconds press on the button on the bottom right corner until you hear a beep (see ;How to reset Apple HomeKit on my Smarther AC with Netatmo connected thermostat? ;).