I had a similar problem recently in the other recent cold snap although on the near side front door. Mine was stuck shut and the door couldn't be opened. The fix involved removing the door card which is definitely harder with the door shut.
It is possible that with the cold weather and doors being harder to open that the rod connecting the exterior handle to the lock mechanism on the inside of the door can become dislodged or can change position to the point that it isn't functioning properly...It is basically a rod with a not so impressive white plastic clip on the end which then connects to a lever on the actuator. With the door card off you can reach inside the door and push this lever down yourself to check it works (make sure the car isn't superlocked - the lever wouldn't open my door until I tried with the engine running... I have no buttons on my fob to unsuperlock). Then you can reposition the placement of the rod within the clip until you have a working door.
I am not great at explaining things. Hopefully someone else comes along who is better
Disclaimer: having not seen the interiors of a rear door I'm not 100% sure if the set up is the same as the front