Since iOS doesn’t make it easy to run a flush DNS command, I’ve discovered turning airplane mode on and off will clear your cache.
I do see less ads in apps with NextDNS. However, I do run into scenarios where certain apps don’t work. For example I know if I want to use DoorDash or Facebook, I need to turn off blocking and flush my DNS cache to run those apps. I don’t see this being a viable option for a nontechnical user.