You log into your router at its base address:
mine is at 192.168.11.1 but most are at 192.168.1.1. You just enter that in your browser address bar.
That takes you to a login page from your router.
Once logged in there'll be a number of configuration tabs, here is the relevent one from my Buffalo router:
The DNS is set here as Google primary and FreeDNS as secondary.
As a precaution I only allow router configuration from a wired Ethernet connection. This is set within the router on another tab.
DNS config.JPG
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History is much like an endless waltz: The three beats of war, peace and revolution continue on forever.
However history will change with my coronation - Mariemaia Khushrenada