the mobile device and the laptop must be connected to same wifi when using WebRatio Mobile Developer App.
The reason is that WebRatio Mobile Developer App connects to the PhoneGap server hosted on your laptop, and in order to do so, it must use the same network to reach the laptop local ip address.
The Backend URL is another property that refers to the remote location of the Backend application.
Do not confuse with the connection between mobile device and laptop.
If the Backend is published on a public server (like it happens with the WebRatio cloud servers) then it can be used from the app when its built but also when its emulated through WebRatio Mobile Developer App.
If the Backend is published on your laptop then it works only if the connection between the mobile device and the laptop can be established.
I hope now everything is clear.