Sign in with the owner email and password. The same credentials manage every enrolled phone.
On the phone open MDM Guard → Settings → Connect to dashboard, enter this server's URL plus this same email & password. It appears here automatically within a minute.
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Turn off all protection releases the DNS and every restriction but keeps the app managed (reversible).
Remove device owner makes the MDM app uninstallable on the phone. It runs on the phone's next sync and can't be undone from here — you'd re-provision with adb to manage it again.