A factory default FireBrick configuration will work with a ZTE MF823 dongle without any additional configuration

The FireBrick will DHCP an address in the 192.168.0.x range from the ZTE dongle
You can access the dongle's web interface via http:192.168.0.1
There will be NAT performed by the FireBrick and then again by the dongle

Changing the ZTE MF823 to 'modem' mode

Warning: This is still in progress and does not work

The ZTE823 works as an Ethernet device by default, it is possible to change the mode so that it works as a modem. The idea here is that rather than the dongle NATing the connection it can be a modem and the FireBrick will be given the public WAN IP address directly.

Mode switching

Disable auto-connect
- Go to http://192.168.0.1
- Click on Home
- Click Disconnect
- Click Settings
- Click Network settings
- Click Dial-up settings
- Click the radio button labeled 'Manual'
Put the Dongle into download mode:
- In your browser, go to: http://192.168.0.1/goform/goform_process?goformId=MODE_SWITCH&switchCmd=FACTORY