If you use pihole, you just need to forward port 53 to block the fallback dns built into android. Even blocks ads in games and other apps.

I spent a long time trying to figure that one out, but the port forward worked like a charm.

And while you can’t take the pihole with you, many routers support creating VPNs so you can stay on your home network all the time to use the pihole.

Here’s a Reddit post about it, though it’s more about firewall stuff it has good info about how it works.