Not a solution for your question, but worth giving a try, the very extensive community ‘tadoZones’ app → click here
You can install it alongside Athom’s tado app to check it out.
About your Q: Commanding the tado thermostat to switch the heating on will not have any effect, when the room temperature of the tado thermostat is at target temperature.
Only with tado valve controllers (thermostat taps), this is possible.: When one room needs to be heated, the thermostat will automatically engage the heating, you just have to set the target temperature of that specific room with Homey.
You’ll need to put valve controllers in the thermostat room as well, otherwise that room also gets heated, when only (an) other room(s) demands heat.
