Good question, I forgot this happens time to time. I set DNS at the router instead of devices, so just tell them to turn off wifi on their phones when that happens. It's actually slightly more complicated because of parental controls(if you care)... essentially the router gives out its own IP for DNS via DHCP, and the router itself is configured to use controlD.

On my personal computer, I don't remember ever running into this, but if I did I'd just override resolv.conf temporarily.

You can also just whitelist the domain(s) too via oneclick actions in both systems, which was my initial caveat that you can't do that using public adblocking DNS.