Use Google's public DNS servers.

nameserver 8.8.4.4
nameserver 8.8.8.8
But things change and now it’s not that simple. If you now edit /etc/resolv.conf on Ubuntu you’ll find that the edits are ephemeral. If you restart (or even hibernate) your machine then they’ll be overwritten by default content.

nameserver 127.0.0.53
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This is pretty simple to fix though.

Install the resolvconf package.

sudo apt install resolvconf
Edit /etc/resolvconf/resolv.conf.d/head and add the following:

Make edits to /etc/resolvconf/resolv.conf.d/head.

nameserver 8.8.4.4
nameserver 8.8.8.8
Restart the resolvconf service.

sudo service resolvconf restart
Fix should be permanent.