DIAGNOSIS It's usually one of three things: the remote's IR LED is dead, the sensor window on the AC unit is dirty/blocked, or there's strong direct sunlight or another IR source (like some LED bulbs) flooding the sensor.
FIX Test the remote with your phone camera. Point the remote's LED at your phone camera and press a button. You should see a faint purple light on your phone screen. If you see nothing, the remote is dead. If you see the light, clean the small, dark plastic IR sensor window on the AC's control panel with a dry cloth—it can get dusty and block the signal.
IF THAT DIDN'T WORK Try operating the unit with the remote in a different room, away from windows and bright lights. If it works there, you have IR interference at your usual location. You may need to adjust the angle of the unit or shield the sensor from direct light.
HEADS UP If you need a replacement remote, the model number is usually on the back of the dead remote. You can order an official Frigidaire replacement part. Universal remotes rarely work well with these units because of the specific timer and mode functions.