→Changing the ZTE MF823 to 'serial modem' mode
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.
One caveat of modem mode
, is that it does not seem to reliably respond to AT commands, as it appears the dongle is generating its own AT commands internally, presumably part of its web interface thing. It does appear to respond to the AT+CGDATA command to start a data session, with almost no other interaction required on the behalf of the Firebrick.
==Switching to 'Download' Mode==