- Is this your corporate/organisation/enterprise/School Management iPhone? Then please contact the system administrator of your corporate//organisation/enterprise/School Management
- If it is your device, in that case, do you have any profile installed by your corporate/organisation/enterprise/School Management?
- Erase the profile added by you If you have installed a profile on your own --> Install or remove configuration profiles on iPhone - Apple ...
- Or please contact the system administrator of the organisation who provided the profile to you

It’s typically something only seen in debug menus, carrier profiles, or enterprise Wi-Fi settings (MDM). End users usually don’t toggle it.
On iPhone Wi-Fi, DNO = Disable Neighboring Offload, a power/roaming management setting buried in system networking.