I have also just moved to VMB, as far as i can gather the SIM PIN must be random, and only the config on the Hitron router knows what it is.
I have found after being lied to by virgin media that indeed it does NOT kick in when the Hitron router is in modem mode, as it has no means to, the dongle would need to be in your own router in that scenario, but you cant as its got the PIN... but it may well fail over fine in Router mode (if the sim is connecting ok)
No one at virgin seems to understand this and it does seem rather scammy of them, seems they are all told to say "it will in an outage" but dont seem to realise that pulling the coax out is the same as an outage, and the problem is that it just doesnt work in modem mode, at all.
You could, in an outage, switch back to router mode and have config ready to apply (if it doesnt hold it between modes) to get around the double NAT the best you can. Alternatively, get VM to cancel the SIM part of your deal as its clearly useless to anyone using a business connection, because who on earth uses the ISP's router? its not even nicely rackmountable...
Keep the Huawei USB and stick it in your own router with your own sim card in it.
Personally, ill think ill be on the phone to them soon getting rid of this useless sim, clearly they just want to get their sim issueing numbers up with 02 now they are in business with them.