AirCard 785S keeps dropping connection due to APN settings

I have an AirCard 785s that was originally locked into the Australian Telstra network. That network went down yesterday due to contractors digging up the backbone cable for the towers in this remote area. I switced out the Telsta SIM and inserted the Optus SIM from my phone, entered the unlock code and it worked momentarily. I then realised it needed an Optus APN code, and as Optus has a few I created a few entries. They work for a few minutes then the connection drops. If I switch to another APN in the Netgear admin interface it restarts again, even if I select Telstra APN. It seems it just needs to be toggled every few minutes to restart the connection but of course that's a pest. Any idea what's behind this? I've restarted twice in the time its taken to write this.