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