iPhone keep changing the configuration

I'm trying to keep my son's iPhone configured with NextDNS, but the phone keep changing the configuration back to the iPhone's default.

After setting back the configuration to NextDNS, just a minute after it change back to the default. As I could see on the image, after the address with dns on it, it didn't answer anymore. So, I was looking again on configuration and Iphone's default DNS was back.

How can I can keep the nextdns configuration?