Diagnosis
The infrared receiver on the robot is obstructed, or the remote has lost its pairing code. The remote uses IR, not radio frequency.
How to Fix It
Ensure the robot's IR receiver window (a dark plastic panel on the top front) is clean and unobstructed. Point the remote directly at it from within 3 meters. Press and hold the 'Auto' and 'Spot' buttons on the remote simultaneously for 5 seconds. This re-initializes the command set.
If That Didn't Work
Test the remote's IR emitter. Use a smartphone camera. Point the remote at the camera lens and press any button. You should see a faint purple light through the camera screen. If not, the remote is faulty.
Heads Up
The robot can operate fully from its top panel buttons. The remote is for convenience only. All cleaning modes are accessible manually.