Hi, lets say you were trying to configure DDNS in the Settings->
Security->DDNS in your brand new Modem. Select your service provider, set the status to ON. Type in your chosen Domain Name, User Name and Password and save it. I assume that User Name and password are correct. The modem will not complain. But no connection :( Probably your password contains special characters. This could be a possible solution.
Change your password and remove special characters and retry.