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

ZTE MF823 LED Status
Solid red	On, but not connected
Solid green	Connected to 4G
Flashing green	Downloading data on 4G
Solid blue	Connected to 3G
Solid green	Downloading data on 3G

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.

Switching to Modem Mode

Disabling Autoconnect

Soe of this information came from: archlinux.org

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
- The modem should respond with: {"result":"FACTORY:ok"}
Unplug and plug back in to the FireBrick and the FireBrick should see the dongle as a modem
TODO - HOW TO CONFIGURE THE FIREBRICK TO USE THE DONGLE

Switching back to Ethernet mode

To put the dongle back to Ethernet mode you will need to connect the dongle to a computer in order to send it some commands via a serial terminal

gtkterm

Open gtkserial or another serial terminal (or simply echo the following AT commands to /dev/ttyUSB2) (or maybe another dev/ttyUSBx):

ATZ
AT&F
AT+ZCDRUN=9
AT+ZCDRUN=F

Disconnect the modem from the computer and reconnect it to the FireBrick

What mode is it in?

On a linux system, if you run lsusb you'll see something like:

Bus 002 Device 030: ID 19d2:0016 ZTE WCDMA Technologies MSM

The 0016 is the mode:

1225 – Default Mode. Available USB Mass Storage Device with CD-ROM and card reader. Corresponds to AT+ZCDRUN=9+AT+ZCDRUN=F
1403 – Operating Mode. Available RNDIS adapter and Mass Storage Device. Corresponds to AT+ZCDRUN=8+AT+ZCDRUN=F
1405 – CDC Ethernet Mode (the one we need). A mode similar to that described above (1403). Included in Linux after starting usb_modeswitch c default settings.
0016 – Download Mode. Under the name of ZTE., but simply a mode where available diagnostic port and two command (analog modem port and PC UI devices Huawei). Corresponds to AT+ZCDRUN=E
0076 – "real" Download Mode. Includes a standard for devices running QC methods.