It is now working.
It was shipped with a SIM that needed to be ACTIVATED.
At least it wasn't a faulty SIM.
Did the SIM come pre-installed, or did you have to install it your self?
It did. Nothing "obvious" happened until I unplugged the NBN, and then the "Signal" and blue "Network" lights came on and the backup was working.
That's normal. The network and signal lights only come on when actually trying to use the 4G backup, otherwise they are always off.
I'd recommend setting yourself monthly calendar reminder for the next 2-3 months to test the backup. As it didn't work straight out of the box as it was supposed to, I'd be wanting to make sure that its actually working as intended.
> Did the SIM come pre-installed, or did you have to install it your self?
> As it didn't work straight out of the box as it was supposed to,
As shipped it couldn't work "straight out of the box". Everything I can find implies it should, including connecting via 4G before your NBN is working:
But my SIM required activation. I suspect others might as well, but won't find out until they need it.