NextDNS has not updated its client applications on multiple platforms (iOS/iPadOS/macOS) for several years. Those client applications did have the ability to stop the blocking (or not), but now it's just a toggle that does nothing.

Most of the time when I visit test.nextdns.io it shows as "unconfigured" even though the NextDNS client is installed and configured with a NextDNS profile (and approved in Settings as a VPN provider on these OSes). Sometimes it will work on its own.

I wouldn't recommend NextDNS unless the user is comfortable installing a (somewhat) permanent Profile on these devices with no temporary "off" switch to stop blocking. For me it's important to stop the blocking once in a while.

At least on macOS, there's Little Snitch (paid application), which can subscribe to the same blocklists used by ad blockers and has a working toggle.