Smart Thermostat not switching off

Hi, i recently installed a Smart Thermostat V3 downstairs in my home. Works really well and i have linked it apple home and can use either app to control it. I decided to add a secondary smart thermostat to my upstairs heating circuit. It’s all connected and the temperature can be controlled remotely, at least on the device. But it doesn’t actually switch off when up to temperature or manually turned off. As soon as i plug it in the heating comes on, i am thinking it is faulty, i tried it downstairs and it does the same. I am though wondering if i have missed something simple in the set up, and was wondering if anyone else had experienced this issue ? Thanks.