VW Carstick connection problem

[Tapnstrum]

I've bought a VW carstick and inserted an activated 1p mobile (EE Network) sim into the stick. Hey presto; within seconds of starting the engine the carstick glowed green and the VW 3G icon in the navigation screen illuminated. I assumed that I had cracked it. Unfortunately subsequent to this initial success I have rarely managed to re-connect. The sim remains active and the green light on the Carstick remains solid green. I know that it is working because I can connect to the carstick WLAN using my phone. The satnav (discover navigation) however seems to re-connect only very intermittently and apparently fairly randomly. I have tried manually entering and re-entering the APN settings which sometimes seems to trigger a temporary re-connection but the same problems recur as soon as I turn the engine off and restart the van again. Is this a common issue or am I doing something wrong?