If you still want to set up OpenVPN manually, go step-by-step through following instructions: OpenVPN Setup instructions. You have to connect to your DD-WRT router using Ethernet cable, or Wi-Fi network.
In the DD-WRT Administrative Interface, navigate to Setup > Basic Setup. Under Network Address Server Settings (DHCP), set these NordVPN DNS addresses: Static DNS 1 = 18.104.22.168 Static DNS 2 = 22.214.171.124 Static DNS 3 = 0.0.0.0 (default) Use DNSMasq for DHCP = Checked DD-WRT Setup – HMA Support Nov 18, 2019 DD-WRT on ASUS RT-N66R Instructions - YouTube Jul 06, 2014 Using DD-WRT with OpenVPN Access Server | OpenVPN
The Wiki is mainly maintained by the DD-WRT community and contains a lot of information, often in several languages. Because DD-WRT has matured over the years take care to check the respective wiki page what version of DD-WRT a manual is written for. In some cases the setup and behavior of DD-WRT differs between the releases.
He/she did mention "if you have dd-wrt fw installed on your router," but being completely new to the subject, I was left to assume that the instructions therein should be available on some OpenDNS screen. How To Set Up A Repeater Bridge in DD-WRT - FlashRouters Feb 19, 2018
DD-WRT on ASUS RT-N66R Instructions - YouTube
DD-WRT Setup – HMA Support Nov 18, 2019 DD-WRT on ASUS RT-N66R Instructions - YouTube Jul 06, 2014 Using DD-WRT with OpenVPN Access Server | OpenVPN Introduction Many of our users have expressed interest in using DD-WRT or related routers to connect to VPN servers hosted behind Access Server. While using OpenVPN in this manner may not yield the best performance, due to the limited processing power and memory of the router, it could be useful in the cases where convenience, […] How to Flash Your Router With DD-WRT Firmware