Key fob stopped working
 

Hello all, I have 2009 C300.
couple days ago when I was trying to unlock my door, the remote didn't work and when I pushed the unlcok button, the red light stayed on, I removed the battery for a second and then tried and it worked. Yesterday, the remote didn't unlcok again, but the red light didn't stayed on, I removed the battery, didn't fix the problem, manually opened the door, when I inserted the fob, the monitor showed: key not belongs to the vehicle.
i found my spare key and it works fine.
Things that I've done so far:
1- changed the fob batteries,
2- put the battery slots of the bad fob to the good one, the bad one is still bad and good one is still good.
3- the red light and infrared( seen with a camera) both light up when I press a button.

Any suggestions!?
Thanks