I've just had this issue, with a 2002 Megane hatch, it may help you out.
Couldn't open the rear offside door. It may also be good for other Renault models if they use the same door mechanisms.
The reason I couldn't open the door :- Child lock was on, external handle connection broken.
First point of attack, I found it here on the forum, is to release the child lock. You can use what ever comes to hand but I cut the longest side out of a 1 litre plastic milk carton. You need something akin which is fairly flexible, about 40 cm long and 20-30 cm wide.
Now wind down the window of the offending door, and, from the outside, slide you piece of plastic in as far as it goes, starting from just below the window. Once inserted, try the inside door handle, work your way down the door until released. It shouldn't take more than 2-3 goes.
I have a 2001 Megane.
Not quite sure what happened, but my drivers side rear door has failed to open from the outside.
You could still open the door from the inside. Me being a fool, I thought this might be the child lock (not thinking) so put the child lock on and now the door is also locked from the inside.
So now I have a door which I can't open!
I have tried your technique with a 2l coke bottle cut into a large flat strip.
I placed this between the window and the outside panel. Tried different parts of the window, up and down trying to release the child lock but with no joy. Also tried briefly on the other side of the window.
Could you give some further instructions/guidance on how you released the door? Am I missing something?
Otherwise, is there any way I can take off the door panel without taking the back seats out?