Sorry i do not use “ddclient”. But i think you need external ip and not internal ip.
Please read:
https://superuser.com/questions/389125/ddclient-updating-to-local-ip-instead-of-public-ip/389276
This is your external ip: http://checkip.dyndns.org
You must configure port-forwarding on your router that it works from outside.
Also you can configure dnsdns often direct on your router e.g. FritzBox.
note:
Sometimes you get problems if you access from intern your intern nextcloud with extern name or extern ip.