Firstly, check the file /etc/resolv.conf and see what's inside. Make sure it only contains the DNS servers you want it to contain. The NordVPN servers are:
nameserver 78.46.223.24
nameserver 162.242.211.137
If there are any more, delete them, and then make the resolv.conf file immutable with the following command:
sudo chattr +i /etc/resolv.conf
Reboot.
To reverse the process:
su
chattr -i /etc/resolv.conf
nano /etc/resolv.conf
Enter preferred DNS servers and make file immutable again:
chattr +i /etc/resolv.conf
