I don’t use WD Sync but I would say you missed something if you read the User Manual about WD Sync.
The way I read it you have to have WD sync on all the devices you want it to work on and there is a specific folder that the files have to go in. See excerpts from User Manual below.
After installing the WD Sync software, to sync a file or folder on your computer, use your computer’s file management utility to copy and paste or drag and drop it into the WD Sync folder. The WD Sync software automatically copies it first to the WD Sync folder on your My Cloud device and then to any other computers connected to it. After that, the WD Sync software automatically updates any changes to the file, at any location, on the other configured devices.
Copying Into the My Cloud Folder When you copy a file or folder into the My Cloud folder on your computer, the WD Sync software automatically copies it to the WD Sync folder on: Your My Cloud device Any other computer in your network with the WD Sync software installed. Then, whenever you change or delete a file in the WD Sync folder on any device, the WD Sync software replicates the change on all of the other devices.
WARNING! The WD Sync software is designed for data files. It is neither intended nor suitable for syncing your applications or executable program files
I suggest you go back and read over the information in the User Manual again.