sudoers can not edit network settings

I have two users which I have defined as sudoers in /etc/sudoers which looks like this:

Defaults        env_reset
Defaults        mail_badpass
Defaults        secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin"

# Host alias specification

# User alias specification

# Cmnd alias specification

# User privilege specification
root    ALL=(ALL:ALL) ALL
bob      ALL=(ALL:ALL) ALL
mary      ALL=(ALL:ALL) ALL
# Members of the admin group may gain root privileges
%admin ALL=(ALL) ALL

# Allow members of group sudo to execute any command
%sudo   ALL=(ALL:ALL) ALL

However after entering their password, none of them can change Network Settings to add manual DNS IP addresses. Instead I get this annoying message:

System policy pervents modification of network settings for all users.

I have no difficulty switching to root using sudo su in the terminal.

So how can I fix this?