How to use the VThermo app with Mill

Hi! Newbie here. I’m trying to get the VThermo app to control a Mill heater in the same zone. As I have understood I just have to place them in the same zone and everything will work. I’m using a Netatmo sensor, and that’s working well, but there’s no connection between VThermo and the Mill heater. What is it that I don’t understand here?