http://cccpastefzuz6unowzgjtdwmxwjfgf5za7owwci3b2zys4jmc3xjszad.onion/cccea7c18ebdb7dd
The Image Builder can run only in a 64bit Linux operating system, so if you don't have a linux system on hand, look up a tutorial to install Ubuntu 64bit in VirtualBox. Then go in the same download page where you can download the firmware for your device and scroll down until you find a file starting with “OpenWrt-imagebuilder”.