Goto this link https://developers.cloudflare.com/1.1.1.1/setup/

Expand Block Malware and copy first two IP Address

Goto router's admin panel and put this two IP on DNS1 and DNS2 and Save

To test if it's successfully set or not visit https://my.nextdns.io/start

If successfully set you'll see This device is currently using ”Cloudflare” as DNS resolver.

Or use this link https://1.1.1.1/help