I think AT&T's fiber modems are using 1.1.1.1. I'm getting < 1ms ping times and according to Cloudflare's website there's no data center close enough to me for that to be possible without violating the speed of light.
what happens if you go to https://1.1.1.1 in a browser? It should have a valid TLS cert and have a big banner that says, among other things, "Introducing 1.1.1.1". If your ISP's CPE or anything else is fucking with traffic to that IP, it wont load/display that
Call your ISP and ask them why they're blocking access to some websites. Ask them if there are any other websites they're blocking. Tweet about it. Etc