Thanks guys! That link solved my problem. Adding the dns-record to the "network" config file solved my problem. I added both NordVPN DNS servers to my NordLynx interface:
option dns '103.86.96.100 103.86.99.100'
After a reboot, everything worked fine.
I guess that was what the topic starter meant with "adding a dns"... 
It seems a little strange that you can't add it through the LuCI interface though...