Follow instructions on your router to log in to it. Usually typing 192.168.1.254 or similar in your browser.
DNS settings are like the “phonebook” your router uses to look up sites on the internet. By default you are using whatever your ISP decides. A lot of people use Google’s DNS.
A company called AdGuard makes apps for blocking ads on device (Mac and iOS works well, but eats up ram, etc).
A better solution: use their DNS settings to block all ads from coming into your house, period. No more using adblock apps on device. Just gotta put those numbers in your router’s DNS settings, in the WAN section. You could also set individual devices’ DNS settings but why not do whole home? There is also a “family” version that will block porn, etc.
https://adguard.com/en/adguard-dns/overview.html <- for more info