In theory it should be just the same as the femtocells are for mobile phones - A VPN back to their network over an untrusted connection.
It's all for nothing anyway, I just got off the phone with them and they can't send out a Hybrid Connect unit as the computer says no - The signal in the area is too poor. (Even though they automatically send out a mini hub which doesn't support external antennas.)
In my experience so far, the Mini-Hubs are are much better at connecting to the 4G service than the Hybrid Connect units. The Hybrid Connect unit shows Flashing Red everywhere in my property, including on a balcony with direct line of sight to an EE mast, whereas the Mini Hub always shows 3 bars and achieves a download speed of about 30 Mb/sec.
Oddly although the device is showing Flashing Red, the Hub Manager is showing 2 bars with a signal strength of -104dBm.