ASUS openVPN Client to ASUS openVPN SERVER

I have 2 of the asus routers ... how can I create a site to site VPN connection...
I tested the ASUS Merlin openVPNserver already using the openvpn client on the MS side and it tunnels perfectly (tap)
But how do I...
1. setup the ASUS VPN client1 side router....
2. do I use the WAN port to connect the client router to the internet.
3. Does WAN need to be changed from automatic to L2TP...

I cant find any tutorial to get that to work (ASUS Merlin VPN client side)

Thanks in advance for your help ...