DNS hijacking

Why is xfinity blocking me from using the internet when i change to certain dns servers such as alternate-dns or adguard. I researched and came across an article from around 2009 that mentioned something about opting out of their “domain helper”. Ive searched everything and read every policy on xfinity’s website and i cannot find anything of the sort. If they could stop trying to control literally everything that would be great as i have already opted out of all of their marketing [Edited: "Language"]! This infuriates me. It isnt enough they have completely monopolized making it virtually impossible to choose other providers by exploiting loopholes in the states laws, but now they still try to track everything despite opting out. It is getting a bit ridiculous. Apple users are already seeing they block encrypted dns traffic. Get it together xfinity. People are entitled to some amount of privacy whether you like it or not and by injecting your [Edited: "Language"] into all of your traffic you are only allowing vulnerabilities for hackers.