Hi rbroome,
When connected over 3G, MobiLinc needs to know how to route it's requests back through the internet to get to your router which in turn needs to know how to route external requests internally to your ISY. This process is called port forwarding and we have a general outline guide here that explains what needs to happen with MobiLinc and your router to make this work:
viewtopic.php?f=6&t=123Wes