If you mistype a hostname in the browser and DNS lookup fails, they return a valid IP for a landing page with an ad. AT&T doesn't do it for my service, but at some point I had it with Time Warner cable, IIRC. You fix it by setting your DNS to (haha) Google's.
Try 9.9.9.9 and 1.1.1.1. There are many other external DNS services that don't break AD and eDirectory. You can also run your own but I'll grant you that isn't for everyone.
It does a yahoo search for me which includes ads if I use their dns servers.
But yes, this is why you use the DNS service of your choice instead of whatever dhcp returns.
I don't have uverse, so I don't know how they implemented it. I'm guessing it's done by violating DNS specifications.
Probably an obnoxious https mitm certificate that adds banners to your web browsing, along with a complete recording of your browsing history to sell to advertisers, like comcast does.
You're saying you can't visit any https sites without adding this certificate to your device's trusted certificates?