Asus GT-AC2900 Faq page 5

How to set up VPN server with port
forwarding?
How to set up VPN server with port forwarding?
When WAN IP address of your router is found to be a private / virtual IP address, your router
may be in a wireless network as shown in the image below. Your router connects to Internet
through another router (here we called "the Root AP"). The Root AP assigned IP address to
your router. In such case, you have to set up Port Forwarding, NAT Server, or Virtual Server
on the Root AP to connect to your router over Internet via a VPN.