The input of DNS addresses are required by Nordvpn per setup guideline back in Feb 2018. I verified that there is no DNS address in any of the Nordvpn configuration files I am using for VPN servers up to us2000.nordvpn.com.tcp, if I don’t input the Nordvpn DNS address, ipleak.net spews out leak error.
Embedded DNS in the config file may be true for newer servers, and Nordvpn guideline for GL-iNET product has skipped the DNS address step within the last month or so, as compare to the guideline I followed back in Feb 2018 that told me to input the DNS manually.
Regardless of how you get the DNS address, I recommend you run ipleak.net test to make sure you have no leak.