It only requires that you remove all of the existing DD-WRT changes, and then paste a script into the DD-WRT router. Please take a look at THIS PAGE where you will find the instructions you'll want to follow to get the script working. It looks like a lot to do, but in fact most of it is just a script that you copy/paste into DD-WRT.
2010-12-24 DD-WRT firmware, username/password combination wont … 2019-9-2 Reset And Reboot - DD-WRT Wiki
DD-WRT v2 Router Setup – IPVanish
Resetting Username and Password in DD-WRT on NETGEAR
If you don't see the Username and Password fields, that means you have an older version of DD-WRT which requires additional steps. Step 9 In the Additional config, add this line:
DD-WRT NordVPN scripts | DD-WRT_NordVPN 2020-7-1 · DD-WRT NordVPN scripts Synopsis. The purpose of this project is to enable a router liberated by DD-WRT to connect to the service NordVPN.All the clients in the network attached to the router will be able to use NordVPN via one login.