Custom DDNS update script on ASUS router with merlin firmware

This post is a bit different than others, but it might save some of you some time. Lately, my ISP upgraded my network connection and forced me to use new cable modem which turns out to be multifunction modem and router. Unfortunately, it turns out that it had buggy firmware and was unable to work in bridge mode. It was very shitty device, so I would like to use my own ASUS router.