Routers not recognizing DNS

I had the same problem with my Netgear R7000. I could not change the DNS. I even disconneted the router from my modem, no go, then I changed both of the the DNS to 8.8.8.8, that set, then I went and changed it to what I wanted, and then it took. The Modem was disconnected from the internet when I did the change that worked, don't know if that affected it.