DIAGNOSIS: The remote's IR signal isn't reaching the unit's sensor, or the sensor is blocked/faulty.
FIX: First, verify the remote works. Point it at your phone camera and press a button. You should see a purple light flash on the phone screen. If not, the remote is dead. If it works, clean the IR receiver window on the AC's control panel (a small dark plastic circle) with a dry cloth. Ensure you're pointing the remote directly at it from within 10 feet, with no obstructions.
IF THAT DIDN'T WORK: The IR receiver on the main control board may have failed. You can still use buttons. A board replacement fixes this, but it's often not cost-effective.
HEADS UP: Using aftermarket or rechargeable batteries can sometimes provide insufficient voltage for the IR LED. Use name-brand alkaline batteries only.