Asus RT-AC66U - Manual (Page 77)

4.3.6 NAT Passthrough
NAT Passthrough allows a Virtual Private Network (VPN)
connection to pass through the router to the network clients.
PPTP Passthrough, L2TP Passthrough, IPsec Passthrough and RTSP
Passthrough are enabled by default.
To enable / disable the NAT Passthrough settings, go to the
Advanced Settings > WAN > NAT Passthrough tab. When done,
click Apply.