Many routers do not let you specify DNS servers located on the LAN network that they host. The router enforces an upstream DNS provider that it can access via the WAN. Unfortunately in many cases these same routers have no option to change the DNS used on the local network.
You're in luck though because your Netgear C6300 allows you to disable DHCP service (which is how DNS addresses are provided to the networks). On the C6300, the settings to do so are in "LAN Setup", where you would have to uncheck "Use Gateway as DHCP server".
Your Pi-Hole can instead serve as your network's DHCP server, as well as DNS.
Check the FAQ for how to enable DHCP on your Pi-hole
And on your router, you can leave the DNS as "Get Automatically from ISP" as it will not be used by anything else on your network anyway once the Pi-hole takes over DHCP.