I'm on a phone only in the middle of France at the moment. Try searching for a YouTube video on fitting "Proplates" to.a Citroen Relay. It shows the removal of the driver's door lock and handle from the door skin. The back door is basically the same although there may be an extra stud and nut securing the handle, I can't remember offhand. You should also be able to see how the actuating cable connects. There is only one on the front door handle, but two I believe on the rear and side doors. They connect to the locks and operate in a similar way to cycle brakes, with a fixed outer sleeve and an inner pull cable. One of these cables having come adrift at either the handle or the lock end is almost certainly your problem.
Search for Ducato/Relay/Boxer lock handle on Google as well. Select images and you should find some clear photos of the back of the handles and actuating mechanisms.
eLearn for the Ducato X250 will describe the procedures with diagrams. You can download it from the download section on this site, or there are eBay sellers offering the Citroen Relay workshop manual which also describes the process and arguably has better illustrations since they are photos of the actual assemblies rather than line drawings.