DIAGNOSIS The "C" (common) wire is missing or not providing power, forcing the thermostat to run on batteries alone, which isn't sufficient for Wi-Fi operation.
FIX This isn't a battery problem—it's a wiring problem. Check that you have a "C" wire connected at the thermostat's wall plate. If you don't, you need to run a new 5-wire thermostat cable from your furnace or install the Lux Power Extender Kit (PEK) at the furnace to create a "C" wire substitute.
IF THAT DIDN'T WORK If you *do* have a "C" wire connected, the connection might be bad at the furnace control board. Turn off HVAC power, open the furnace panel, and verify the blue (or sometimes black) "C" wire is securely attached to the "C" terminal on the board.
HEADS UP The Kono's batteries are only for maintaining settings during a power outage. They cannot power the display and Wi-Fi reliably for daily use. A functioning "C" wire connection is non-negotiable for full operation.