Most ISPs provide their own router which will (by default) use their own DNS servers. They will use this to enforce site bans amongst other things.

Anecdotal of course but years back I noticed my service got really slow sometimes, but speedtest.net reported decent speeds. After running the test my service would be fine… for a bit… until I ran another speed test which “fixed it” immediately again for a while.

It got so bad that I’d be running a speed test every 45 mins or so, which would literally make Netflix etc work instantly.

So tried just doing an nslookup on the domain out of curiosity, and wouldn’t you know it that worked too!