NextDNS on iPhone 6 (iOS 12.5.8) worked once, then after a reboot only ever shows "status": "unconfigured"

I'm trying to get NextDNS working on an old iPhone 6 running iOS 12.5.8. It'll be used as an emergency phone for my child, and I want to use NextDNS to stop the phone being used for entertainment.

Because it's an old version of iOS, I downloaded the last compatible version of the app from my App Store purchase history.

Initially everything appeared to work. I entered my Configuration ID, enabled NextDNS, and https://test.nextdns.io showed:

  "status": "ok"

My NextDNS logs immediately started filling up with blocked domains. It was glorious.

I then power cycled the phone and, since then, I've never been able to get it working again.

I've tried:

• Deleting and reinstalling the app.

• Removing and recreating the VPN profile.

• Using it with and without a Configuration ID.

• Toggling the VPN on and off in iOS.

• Rebooting multiple times.

It's always the same. The NextDNS app says it's enabled, VPN shows as connected, but https://test.nextdns.io always returns:

{
  "status": "unconfigured",
  "resolver": "172.71.177.34",
  "server": "anexia-lon-1"
}

No blocking is performed, and nothing appears in my NextDNS logs.

The strange part is that it definitely worked once immediately after the first install. I have the logs to prove it, but it has never worked again since.

Has anyone seen this before or know how to fix it?