Diagnosis
Signal interference or a faulty IR receiver on the unit's control panel are common causes, not just the remote.
How to Fix It
Stand within 16 feet with a direct line of sight to the unit's front panel. Remove any objects between you and the unit. Test the remote by pointing it directly at a smartphone camera and pressing a button; you should see a purple light on the phone screen if the remote's IR diode is working.
If That Didn't Work
Perform a system reset. Unplug the main unit from power for 5 minutes. This resets the control board and IR receiver. Plug back in and try the remote again. If still unresponsive, the receiver on the main unit is likely defective.
Heads Up
Bright sunlight or intense indoor lighting (like LEDs) can flood the IR receiver, making it unresponsive. Try operating the remote from a slightly different angle or with reduced ambient light.