The DNS modification is slightly off. Some ISPs check UDP packets since they are insecure and will modify query results regardless of the DNS server you are sending to. Mediacom is known to do this for their billing and DMCA systems. They use DNS redirection to assist in MITMing the connection to load their own certificate to your browser. With that done, they can prepend their own Javascript to the response they receive from whatever web server you are trying to contact. That’s how they get their data usage and DMCA popups loaded when you load up whatever site.