How to recover from a "broken" Zyxel firmware

[ATC]

Router won't boot properly, it's impossible to access the system menu and perform a firmware upgrade.

[ATC]

First, retrieve the correct firmware .bin file from zyxel.com. The router will accept any file you upload to it, so make sure you get the correct version.

Using a 9-pin console cable and hyperterminal, minicom or a similar terminal emulator, power-cycle the router and enter debug mode by pressing any key within 3 seconds of power-on.

1. Type "atur" and hit Enter
2. Wait for the "Starting XMODEM upload" prompt
3. Transfer the .bin file using the X-modem protocol
4. Type "atgo" to boot the new firmware