Found this! Seems to have worked
Have you looked here and here?
7800 series emergency upgrade
Set static IP:192.168.1.222;subnet mask: 255.255.255.0; gateway: 192.168.1.254.
Plug PC cable into EWAN port.
Hold the reset button and then power on the 7800 series.
When power LED is orange, you could release the reset button.
Open your Browser and access Web UI
http://192.168.1.254
You could see the page as below:
Then please Browse your FW (1.02d.afw or other version). Press Update Software.
You will see a message as below:
After 2 minutes, you can access
http://192.168.1.254 (Router’s LAN IP).
EDIT : Just reread that you have already tried this, please ignore. Maybe Billion reps here can help.