If anyone is struggling with a free dynamic service then I was able to successfully use dynu dns after contacting their support. 

On the most recent firmware select the custom tab.  In the GENERAL section, the update url is exactly as follows (i cannot put in the https.....as it will then not let me post.  but preface the url below with that)........hmmm will still not let me post so remove the "^" below

api^dynu^com/nic/update?username=[USERNAME]&password=[PASSWORD]&hostname=[DOMAIN]

in the CUSTOM DNS section below that put in your dynu account information.

**on the dynu website i'd further suggest you look at the "IP Update Password" option which allows you to create a secondary password for updates vs using the password from your account.