Welcome to the TPG Community!
I would suggest contacting NordVPN to clarify if this is possible. According to their website, it seems like they discontinued the support for common VPN protocols L2TP/IPsec and PPTP.
Here's a link to their website providing a list of popular routers that are no longer supported: https://support.nordvpn.com/Connectivity/Router/1225872552/Not-supported-routers.htm
I am a little confused.
According to the Nord VPN website https://support.nordvpn.com/Connectivity/Router/1047409322/Setting-up-a-router-with-NordVPN.htm :
"For a VPN connection on your router to work, it has to support OpenVPN. Furthermore, it has to support it as a "Client" (as opposed to a "Server") to allow you to connect it to NordVPN servers."
Under VPN settings for my VR1600v, there does appear to be a configuration option for OpenVPN. By default, the checkbox "Enable VPN Server" is deselected, so by this, I assuming it is operating as a client instead of a server, as specified by Nord.
Does this mean that Nord VPN can in fact be configured on the VR1600v router?