First - set up Time Machine support to OFF. Not required.
Create on your mac file:
hdiutil create -size 600g -type SPARSEBUNDLE -fs “HFS+J” TimeMachine.sparsebundle
600G - max size allowed
TimeMachine.sparsebundle - file name for TimeMachine.
Copy file to your NAS.
Mount the file as disk, You will se disk size as configured in hdiutil command
Setup mount by start up.
Set this disk as destination for TimeMachine of your Mac using next command:
sudo tmutil setdestination /Volumes/TimeMachine
Of course you should use your path and name to mounted disk.
TimeMachine will see limit as configured and will use it for backups using NAS space as required.
This WA could be used always, and we have to no wait solution from WD “specialists” 
