Install Kana Solutions dyndns-updater version 3.1.0.7 or higher which can be found here:
http://www.kanasolution.com/products/dyndns-updater/dyndns-download/
Once installed, go to the installation folder (usually found in c:\program files (x86)) and edit the DynDNS.ini file.
under [Options] in the ini file, add the following line and save:
UseLanIP=1
Restart the dyndns updater application and then when you perform an update, the application will use your local LAN IP instead of your WAN IP.