How to set DNS Server?

This one can be tricky but basically just add this to configuration.nix:

networking.nameservers = [ "1.1.1.1" "1.0.0.1" ];

TIP

You can put in just one DNS address and it will still work.

If that does not work as expected and you are using NetworkManager for your internet then make sure to add this line.

networking = {
    networkmanager = {
        dns = "none";
    };
};