DD-WRT NXT has presets for several Dynamic DNS providers:
If your provider is not listed, you can also use “Custom Configuration” and a custom update URL(most providers will provide those to users).
For the Custom Update URL, you can inject the needed informations from the input fields as following:
Example (pure fictional example URL):
http://[USERNAME]:[PASSWORD]@testuser123.org/update.cgi?hostname=[DOMAIN]&myip=[IP]
Where the following parts of the URL will be replaced during the request with the according fields:
The setting of this field will change which IP will be passed to your Dynamic DNS provider for Custom or other providers.