If you're searching for free DDNS you may find results from ZoneEdit.
That happened to me, and furthermore, my ASUS router has an entry that suggests it has settings (script?) to easily connect to ZoneEdit.
However, when I tried to find the free DDNS on the ZoneEdit website I found it impossible.
Looking for support I found other people looking for the same information.
Their agents pretend they don't understand and reply with irrelevant answers.
And, THE MOST SHAMELESS of all actions, they deleted my account so I can't continue to ask questions.
So my experience with ZoneEdit was very bad!