If you see the following error message in your CPanel/WHM

Sorry, the nameserver ip X.X.X.X cannot be removed since it is still registered with an ICANN register.

This is default setting in CPanel/WHM and you will need to disable whois lookups for the nameserver IP manager via 'Tweak Settings' menu in WHM.

Once this option is disabled, it will allow you to bypass the check and use any IP address for your DNS server(s).

Please login to your WHM and on the top left navigation menu you will see a link for 'Tweak Settings'.

Near the bottom of that list you will see a checkbox option for 'Disable whois lookups for the nameserver ip manager.' or similar text.

Check that box off and save any settings. Once that is done you should be able to modify the settings with no problems.