A. Device part of a business infrastructure:
Just don't change anything; those policy are there for a reason!
B. Consumer device:
1/ If we're talking about proprietary hardware/software forcing your network to use a specific DNS, then you need to provide more details because you should be able to change it.
2/ There is also the case for a malware:
A fresh start is preferable.
Disinfect the system while offline, then back up the needed files.
Reinstall the system on a new/old formatted drive.
With the exception of taking your privacy/security seriously this time.
Cut throat environments!