I am trying to get a connection with my device but not succeeding. I have the device connected with a USB cable, and have tethering turned on. Do I need a mobile signal for it to work? My office does not have mobile coverage. I have been out into coverage with my device and signed into AirDroid, and when I come back to the office, it still says I am signed in. But do I need to have signal coverage to actually make the connection with my PC?