These instructions help you to get a router back to stock firmware including restoring the login/password to admin/admin, The reasons for needing to do so are in this thread.
*Disclaimer* This guide is only intended for people who legitimately need to restore a router that they own. It is not intended to be used on routers leased from ISPs. I have never owned a NF18ACV router, nor been a customer of any ISP that provided one. I did a reset for a friend and this is how I did it.
Use a Trivial File Transfer Protocol application. These instructions use Tftpd64 which is is a free, lightweight, open-source application which includes a TFTP client.
Download Tftpd64 from GitHub and install it on your PC (Tftpd64 if you have a 64 bit computer, otherwise use the appropriate version).
Download the firmware for the NF18ACV from https://support.netcommwireless.com/. There are two versions "NC2-R6B023" (21/09/2021) or "R6B016" 14/05/2018. I recommend the more modern version. The download is a zipped file so after downloading, extract the contents into a folder on your PC.
Connect your router to the PC via Ethernet. You can use any of the yellow 'Ethernet' ports on the router. Disconnect your PC from any other wired (Ethernet) or WiFi (wireless) networks. Let the router settle down to steady state on its lights (green 'Power', both 'WiFI' lit and the connected 'Ethernet' port).
Open Tftpd64 and select the 'Tftp Client' tab.
In the 'Host' box put the address for the router (192.168.20.1).
In the "Local File" box put the location/name of the downloaded firmware. This is the file of the type "EN_Upgrade File" that you have already extracted. You can use the "[...]" button to the right of the box to browse to and select this file from your PC.
Click the "Put" button.
Tftpd64 will show the status of loading the blocks of firmware into the router. Click the 'OK' button when it finishes.
The router 'Power' light will change from green to red and the router will take a few minutes to update the firmware. When it is finished the 'Power' light will turn to green and the 'WiFi' lights will illuminate for both 2.4 and 5 GHz channels.
This loads the stock firmware but the previous ISP assigned password remains. The last step is to reset the router using a paper clip at the rear of the router. This wipes the password and any other previous settings and restores the router to stock configuration. Hold the reset micro button down for 5-10 seconds and wait for the router to reset and return to normal light status (green 'Power', both 'WiFI' lit and the connected 'Ethernet' showing traffic).
After this, browse to 192.168.20.1 and enter the 'Gateway Login Details' obtained from the sticker on the bottom of the router (probably 'admin' and 'admin'). You will now be in the stock router setup pages.