How to connect ex7300 extender to flashed R7000 router with DD-WRT firmware?

Does anyone know how to connect a netgear ex7300 extender to a flashed nighthawk R7000 with DD-WRT firmware? I have the router working wiht openVPN. I just want to connect the extender to it so I have better coverage throughout my house. 

I reset the extender and then hit the wps button on it and on the router. They do not connect. I am wondering if I need to make some setting adjustments in the dd-wrt router software?

Any help would be appreciated.