Some ISPs and services/products do DNS hijacking (poisoning) to force all the DNS traffic to go through their own DNS services. It means that if you enable CleanBrowsing, it won't work until you disable the DNS hijacking.
If you are trying to use CleanBrowsing on Comcast Xfinity and it doesn’t seem to be working, the reason might be their Protected Browsing option hijacking your DNS. It prevents any other DNS from working, except their own.
To fix that, go to: https://internet.xfinity.com/network/protected-browsing and toggle the Protected Browsing to off.
Once changed, wait a few minutes and it should start working. You won't lose any protection as all our DNS filters also block access to malicious and malware domains. You may need to reboot your router after waiting 5-10 minutes.
Avast! Anti-Virus, which includes a "Real Site" protection offering which Hijacks your DNS.
Go to Settings and disable this component for CleanBrowsing to work. You won't lose any protection as all our filters include the security-filter by default (that blocks phishing, malware, etc).