For any users that have the Sierra / Netgear AirCard 320U 4G Dongle it takes a bit to get it going - At least in my country. The device appears to start-up in Lower Power mode by default. The device needs to be taken out of this state by entering a number of AT Commands.
I found a guide here that resolved the issue I was having - Basically, I would plug the device into the Router and there would be no lights - Only a blue light for about a second and then it would switch off. The 750S-EXT would detect the 4/dev ports the USB Modem creates but not make the connection.
The resolution I found was to enter the following AT Commands.
Use the command:
and then either:
to enable, or:
A comprehensive guide on the codes required for this modem can be found here for configuring other information such as region/gps etc - AT COMMAND Sierra Aircard 320u - PDFCOFFEE.COM.
It took me the best part of an hour to find the solution for getting this modem to work - Not it works on my Ubuntu system too!
I do find that each time I reboot the Router - I need to enter the same three AT commands - Not sure if this can be incorporate into the Router firmware somehow for USB modems?