DIAGNOSIS The infrared sensor on the AC unit itself might be blocked by dust, or there's a simple reset sequence that got lost. It's rarely the remote if new batteries didn't fix it.
FIX First, clean the sensor. Unplug the AC unit. Look for a small, dark plastic window on the control panel—that's the IR receiver. Wipe it gently with a dry cotton swab to remove any dust film. Next, try a hard reset on the AC. With it plugged in, press and hold the Power button on the unit itself (not the remote) for 5-7 seconds until the display maybe blinks. This reboots the control board. Now point the remote directly at the sensor from about 5 feet away and try it.
IF THAT DIDN'T WORK Test if the remote is actually emitting a signal. Use your smartphone camera (the main lens, not selfie mode). Point the remote's LED at the camera and press a button. On your phone's screen, you should see the LED light up with a purple/white glow. If you see nothing, the remote is dead. If you see light, the AC's sensor or main board might have failed.
HEADS UP The remote needs a clear line of sight. Don't point it at the shiny display or the vents—aim right for that little dark sensor window. Sunlight or very bright room lights can also interfere with the infrared signal.