Using the key in the driver's door should lock everything including the rear hatch. When you twist the key in the door you should hear the air pump run briefly while the doors and hatch lock. Do you hear the pump run? If not then there is another problem other than the remote. You have replaced the two batteries in the remote I assume?
You can contact Needswings or Mercedes swap shop for new FOBS/keys. It won't cost $1,000.

PS let us know what part of which State you are in so we can better help you.