I have exactly the same problem..
I have My Cloud set up on the home business broadband line.
If I use my win 7, or win 8 laptops or my iphone via the same business broadband wifi it works fine.
However, if I switch any of them to my other personal home broadband line I lose the connectivity.
The frustrating thing is that all of this was working up until around a week ago... I have changed nothing on my end but it now no longer works and I get "Can't connect to this device"!
I have tried deleting the My Cloud icon from the laptop app, and deleting the whole mobile app and readding but to no avail. I have also tried most of the other things suggested on this site including diabling/enabling upnp.
The My Cloud device itself is set up correctly as it was previously working and the desktop app says my account's cloud connection has a port forwarding connection established.
This is extremely frustrating...!!!