Note: I use specifically dyndns.org (now dyn.com), this may or may not effect other services.
When setting up the modem’s dynamic dns updater, I kept getting status: bad auth
I triple checked the authentication details and tried both https on and off.
In the end I had to change the password on my dyndns account to one with no special characters.
Also: The modem only supports one dyndns domain/service. This is unfortunate if you want to keep a few updated to stop them expiring.