I don't think that you'll find anything that will work in the way that you want. What happens with the VPN is that your client is not truly assigned an IP assigned an address on your same network. It's assigned an IP outside of your network which the VPN server within the router then internally routes traffic coming over the separate TUN interface established for the VPN from that IP address into your local network.
So say that your internal network is using 192.168.1.x. The IP assigned to your client by default with OpenVPN on the Asus would be in the 10.8.0.x range. Different IP address space/network. The router also has a 10.8.0.x address on the VPN interface so Fing, et. al. can find the router. But beyond that it won't find anything since such
tools need to be on the same network and don't work over a routed connection.
Not sure whether there's some way to set up OpenVPN to permit that but kind of doubt it. Have to let someone better at such things respond to that.
What you could do is RDP over the VPN and then run a scanner from a machine inside of your network. There also are various network management systems with agents that you set up to run inside of your network which then can report that information out to whatever management platform but that's probably beyond the scope of what you're trying to do.