Do they not glow green either when you change the level (press + or -)?
Even if they don't glow green at this moment, they may be damaged by corrosion (water has penetrated), the red-green LEDs could only shine red (green was defective).
Pull off the button cover: Drive with a sharp knife under a lower round corner and carefully pry upwards. if it does not move, stop it, to newer batteries could be glued. Then you can check the LEDs.