1:1 NAT to a cannot use own VPN
An ASUS router is given a public IP from a pfsense using 1:1 NAT. The ASUS router cannot activate VPN since it is on LAN. It says: "The wireless router currently uses a private WAN IP address (192.168.x.x, 10.x.x.x, or 172.16.x.x). Please configure DDNS service before starting the VPN server.".
When the user tries to activate DDNS it says: "The wireless router currently uses a private WAN IP address. This router may be in the multiple-NAT environment and DDNS service cannot work in this environment.".
Is there a known fix or work-a-round for this?
That's something you'll have to ask in a forum for that ASUS router. pfSense can use VPNs and DDNS when it's behind NAT, if ASUS can't, it's a problem with the ASUS router.
Perhaps you can replace the ASUS firmware with Tomato/Shibby, DD-WRT, or something else with better capabilities.