What can happen is that NordVPN changes their DNS ip without warning, and you either end up with no connection, or in worst case the IP is given to an attacker, and you are now using a DNS that is insecure.
You will have to be constantly checking to make sure the DNS is up to date, even motley is hinting at that “103.86.89.100 and 103.86.99.100 (latest DNS)”
For both convenience and safety, you can let the VPN serve you the DNS ip, and you don’t have to worry about that.
The link i gave you in the other post is just 2 scripts that run on VPN connection and disconnection, changing the linux DNS to the VPN one supplied automatically for you. Scroll to the bottom of that thread for the copy paste solution.