Support appeared in r28078, so you have to build image from svn sources. See build. Select Ralink RT288x/RT305x as Target System and RT305x based boards as Subtarget in make menuconfig.

You should obtain <buildroot dir>/bin/ramips/openwrt-ramips-rt305x-nbg-419n-squashfs-sysupgrade.bin.

Unfortunately, the firmware cannot be installed by factory web configuration utility. Someone may try to build the sources from ftp://opensource.zyxel.com/NBG-419N to check what is needed for it. Open the case, connect a TTL-to-RS232 converter cable (not the standard serial cable!) - see port.serial. I used Nokia CA-50 cable. Port speed is 57600, 8 databits, no parity, 1 stopbits, both hardware and software flow control must be disabled. The router does not power on when serial cable is connected, so you have to connect the cable after powering on. Unfortunately you miss the bootloader (U-Boot) prompt, so wait until ZyXEL Linux boots and issue halt or reboot. When you see U-Boot prompt, press 2:

Please choose the operation:
 1: Load system code to SDRAM via TFTP.
 2: Load system code then write to Flash via TFTP.
 3: Boot system code via Flash (default).
 4: Entr boot command line interface.
 6: To emergency.
 9: Load Boot Loader code then write to Flash via TFTP.

You choosed 2

Put openwrt-ramips-rt305x-nbg-419n-squashfs-sysupgrade.bin to /var/lib/tftpboot and start your TFTP server. U-Boot did not like such a long file name, so I renamed openwrt-ramips-rt305x-nbg-419n-squashfs-sysupgrade.bin to openwrt.bin. Connect one of the router's LAN ports and TFTP server to the same LAN.

Then press Y and enter IP for your device, TFTP server and firmware image file name:

  2: System Load Linux Kernel then write to Flash via TFTP. 
  Warning!! Erase Linux in Flash then burn new one. Are you sure?(Y/N)
  Please Input new ones /or Ctrl-C to discard
    Input device IP (192.168.1.1) ==:
    Input server IP (192.168.1.10) ==:
    Input Linux Kernel filename (openwrt.bin) ==:

U-Boot should sucessfully flash and then boot the new firmware image.

Note: This will void your warranty!

• To remove the cover simply remove the two screws on the bottom. Then just open it.
• The board itself has no screws.

The are 4 pins on the board:

1. RX
2. GND
3. TX
4. 3V3

The Bootloader used in this unit is U-Boot. It is available through the serial interface with the settings: 57600/N/1