Changing room of smart thermostat

wadeo0

Should you be able to assign the smart thermostat to individual rooms you have set up in the Tado system? Seems like this should be possible but on mine the room option just says 'heating' and is greyed out. I can change the rooms of all my TRVs in the normal way it's just the thermostat that I can't assign anywhere. Any ideas why this might be? Thanks in advance for any replies!