I have a Model EX6120 Wifi Extender and a Realtek Router but can't get them to connect

I get access to Netgear wifi and the web startup and it all goes through until i get to the part where the prompt says that they are connecting the wifi extender with my router and then i get an error message that says it can't connect to the server. I've walked through this process with the URL and the Nighthawk app and they each end at the same point.