DIAGNOSIS The remote is pretty basic, so if new batteries didn't fix it, the issue is either a stuck button on the remote itself or the infrared receiver on the unit's control panel is blocked/faulty.
FIX First, check the remote visually. Press each button and see if any feel stuck down or are physically damaged. Try cleaning around the buttons with a cotton swab and a bit of rubbing alcohol. Then, point the remote at your phone camera and press a button—you should see a purple light flash on the phone screen. If you see no light, the remote is dead.
IF THAT DIDN'T WORK If the remote emits a light but the unit doesn't respond, the IR sensor on the AC is the problem. It's a small dark plastic window near the display. Make sure it's not covered in dust or obstructed. Power cycle the AC (unplug for 60 seconds) to reset its electronics. controls on the unit should still work.
HEADS UP These remotes are low-power and don't have a great range. You need a clear line of sight to the front of the unit, and being within 10 feet helps. Sunlight or bright IR light from other sources can also interfere with the signal.