The original method in the commit message may not work any more due to Zyxel / ISP firmware updates. A different flashing method using a custom uboot is described in the topic linked below around post 300. This does require you to open the device and use a serial connection. It also does not allow you to easily go back to stock Zyxel firmware since it changes the storage layout. I've converted multiple EX5601s with the custom uboot method and it always worked.