For the benefit of others I wanted to mention my final solution.
Turns out that getting a USB attached 4G Mifi/Dongle to work on pfsense is tricky, bordering on a fool's errand.
Even if you get it to work there are issues, such a ppp not delivering LTE speeds, and mock-ethernet configurations potentially hanging on boot.
Luckily, there is an alternative, without all the headache:
Just buy a 4G desktop router: https://www.google.co.uk/search?q=4g+router&source=lnms&tbm=isch and connect it to pfsense with an ethernet cable.
Yes, you need to be able to afford it (got a E5186 for £120 off ebay), and yes you need the appropriate number of ethernet ports on your pfsense.
But this approach is so much easier than trying to dick around with obscure settings on a very specific MiFi model.
It's also easier to trouble shoot as you can manage and reboot each device independently. ;D