Setting up dd-wrt router as extender when primary router has username and password authentication

I am trying to extend my wifi using a dd-wrt router as an extender. The problem I'm having is that my primary wifi is authenticated with a username and a password. As far as I can see the dd-wrt router does not have the option to pass a username to another router. Does anyone know of a workaround for this.

Ive tried many online tutorial but none of them address this specific issue.