Sometimes even that's not enough. I've had some questionable kit before that would just ignore the DNS settings fed to it if it thought they were no good, and fall back to something else preconfigured.

pfSense is a wonderful tool for situations like that. Anything intended for local use only here just doesn't get outside at all. Handy for stuff like a fire stick that only needs to be calling up a local media library.

It can also mangle any DNS requests going out to a different server and redirect them to itself instead. You could do this without it with iptables/nftables on a generic Linux box, but pfSense makes it much friendlier.

There are other packages that can do the same, but physically all you need is one piece of hardware as a bouncer that manages connections between inside/outside.