@JorgeST 

It might be a case where the domain didn't completely disassociate? When registering a DDNS name it should just check to see if it already exists and reject it.

You could download Tether, log in to your TP-Link ID and see if the router shows up there as a cloud device, if so delete it, then try to register the same name.

I would recommend contact our support team so they can look in the back end for you if anything can be seen: link

If you have not done so, I would also try unbending your TP-Link ID to the router, then rebind, and then try to register the DDNS. If that still does not work try rebooting the device, and if possible even trying factory defaulting the router in case the configuration went bad.