I was getting the "Authentication Failed" message whenever I tried to connect my router to my No-IP. account even though I knew the username and password were correct. Then after deciding to try use the noip API, realized that the password is part of the update URL and my password contained a %, which is not URL friendly.

Changing my password to a new password without a % sign resolved the issue.

So if you are getting the Authentication Failed error make sure your No-IP password does not contain any URL unfriendly characters and try changing it to only use URL compatible characters.

Also make sure you enter your no-ip username and not your email address, and also do not use the @ character in your password as this will also break the URL.

Hope this helps people having problems authenticating their TP-Link routers with Dynamic DNS services.

Chris