The most common problem when trying to connect over 3G rather than the WiFi local network is getting through your router firewall. Airmote needs access to port 5900, and this is often turned off by default. How to turn on varies depending on the router or firewall software, but one way to check if port 5900 is open, is to use a site like this one. Enter 5900 in the custom port test field and you’ll find out if that port can be accessed from outside your local area network. If not, you have to fiddle with firewall settings.