Asus GT-AC2900 Faq page 8

Example:
If you are using OpenVPN, open the OpenVPN configuration file with a text editor and then
change the address in the remote line to the WAN IP address of Root AP or DDNS hostname, as
highlighted in blue in the image below.
Limitation: If the Root AP's WAN type is PPTP/L2TP/PPPoE, you may have VPN
connection problems.