Renault 1998 My back passenger door would not open for mot?

HI,
Please help. The door behind the driver's seat would not open. My machanic had tried to get the door handle side inside panel but the lock will not buldge. He has suggested to get a new machanism but I am not guarantee it will work plus it cost too much for my old car on a student budget.
3 other doors opens but just this door. It did had a lot of rain last month and there was a little leak by the drivers side roof.
I wonder whether it has affected the remote keys?

Anyone with any suggestion will be much appreciated.
Thanks.