In my case the problem was that I used email address. Instead, I had to specify alternative user ID (without @) to make updates work.
Note that email is a perfectly valid alternative to user ID to log in into dyn.com website.
There is an official API description to perform updates by any web-client (i.e. browser): http://dyn.com/support/developers/api/perform-update/
Apparently, OpenWRT DDNS package also uses this method as there is custom option among list of providers which allows to set the URL.
The idea is to guess which username and password can be used with such URLs:
- Before trying to detect updates, set some random IP address for the hostname using web site UI.
- Open another browser window and type URL to try updates directly:
- If the URL worked, use the specified username and password in OpenWRT DDNS configuration.
- Make sure to reboot the router (i.e. reboot command over SSH) to trigger the update.
As soon as I changed email to user ID and rebooted the router, I was able to see new IP address.