When having the VPN setup on router level, some connection issues could be caused by the version of OpenVPN. Specifically, if you are using version 2.4.x, you should encounter no OpenVPN-related problems.
In case you have OpenVPN 2.3.x, you'll need to either upgrade your router firmware or switch to AsusWRT-Merlin in order to use NordVPN.
1. After you have logged into the control panel of your router, select VPN in the left sidebar menu.
2. Press on the VPN Client tab at the top of the page.
3. Now, next to the VPN profile, click on 'Activate'.
4. Now, head to System Log. You will see a detailed indication of your connection status. Scroll to the top, that's where you'll find the OpenVPN version.