Remotely Access WD MyCloud EX2 Using Finder
Your router needs to forward the required ports to your NAS. 

AFP: 548, 427
SMB: 445

Then you can connect to your NAS the same way you do now, but you'll have to use your EXTERNAL IP. If your external IP is dynamic (regularly changing), I'd recommend using a service like dyndns (which keeps track of your IP and binds it to a domain).