DIAGNOSIS You haven't assigned the remote sensor to the active "Comfort Profile." The T9 doesn't automatically average temperatures; you have to tell it which sensor to use for which mode.
FIX On the thermostat, go to Menu > Preferences > Comfort Profiles. Select the profile you want to change (like "Home"). You'll see "Thermostat" listed under Temperature Source. Tap it and change it to your desired remote sensor's name. You need to do this for each profile (Home, Away, Sleep) individually. Now, when that profile is active, it will use the remote sensor's reading to control the system.
IF THAT DIDN'T WORK If the option is grayed out, the sensor isn't properly paired or its battery is dead. Re-pair the sensor (Menu > Sensors > Add Sensor) with a fresh battery first.
HEADS UP The thermostat will still display its own local temperature on the main screen for reference. Only the control logic switches to the remote sensor. Also, if that sensor fails or loses connection, the T9 will default back to its own internal temperature after a while, which can cause surprise heating/cooling.