The remote key of my Merc W202 1996 C180, does not lock and unlock the doors anymore. The car starts, though.
Is the key shell damaged? If it is, there is a good chance that the buttons are damaged as well. Also have you checked to see if the doors look with the central button?
The central lock button still works fine, the remote was tested by MB here, and certified as working. The vehicle does not communicate with the remote
Do you have a second key to test? The reason why I ask is that the circuit of the key responsible for unlocking the car can be damaged. Even if your car starts fine.