Long press the “Mode” button and the thermostat will lite up to indicate it is active. From there you can quick press the “Mode” button to cycle thru the various modes or features which include “AC”, “Fan”, “Dry”, as well as “Heat”. The attached PDF is the manual for the thermostat.
When you have two thermostats, only one of them controls the furnace. As you are thinking, that's usually (always?) the one in the kitchen.
You say 'no longer'.
Has the 12v fuse for the furnace blown?
Wire came loose behind the thermostat?