Why Isn’t my Tado Heating Automation triggering using my Home app?

Hi. I have had the Tado smart thermostat installed but I’m struggling to get an automation to work in the home app.

I’ve created a scene that will heat a room to 20c and then created an automation for it that it is triggered when the thermostat detects a temperature below 18c but it never turns on.

i can turn on and off the heating in home kit so its all connected, its just the automation that isn’t working