If you get any sort of response from the fob it won,t be synching it will need, on the 202 c class the alarm unit is in the passenger footwell (I think this is the same for a 210) where there is a fuse which can blow or crud up, do your indicators still flash? On a 202 there is a white relay in the boot which the alarm interfaces with to flash the indicators on arming/disarming.
Have you got any other L.E.D.s on the car these all have to work otherwise they can cause immobiliser problems.
Don,t just assume the batteries are ok because it still operates the C/L two separate systems and the immobiliser deactivation appears to be more voltage sensitive than the C/L.
Bypassing the immobiliser is possible via the control unit, but there is a different procedure with a diesel as the immobilser is done via a coded fuel valve apparently.