I am using the free legacy DNS resolvers for Control D. It's been up and down regularly the last few weeks. I have been using it for many years without this kind of issue. I am also having issues with Hagezi's DNS – Pro free resolvers.
Control D server status and support says there's no outages to report and to check with my ISP in case it's blocking it.
I have no issue with other DNS resolver like NextDNS, Adguard, Google, and Cloudeflare.
One thing thing that temporarily works (YMMV) is toggle iPV6 within network adapters.
Source: r/ControlD · by /u/SleepheadAir