On your USG you will find the DDNS Settings in the Management and Network Section. (see Screenshot)
Choose your DDNS Type. (in our Case DynDNS)
Type in your „Username“ and the „Updater Client Key“. If you have a Firewall with 2 Wan Ports don’t forget to choose the right one.
Check if you have the same settings like in the screenshot. Enable the service.
Check online at DynDNS your LOG if everything is working. (direkt LINK to you LOG)