What is Syncthing
Syncthing is an application that lets you synchronize your files across multiple devices. This means the creation, modification or deletion of files on one machine will automatically be replicated to your other devices. We believe your data is your data alone and you deserve to choose where it is stored. Therefore Syncthing does not upload your data to the cloud but exchanges your data across your machines [in this case the WD My Cloud NAS] as soon as they are online at the same time.
Source on github
Tested on PR4100 and EX2Ultra. Let me know if it doesn’t work for you.