Yep, I've been doing vpns too much. Your three router model will work fine, if you use a full router rather than a firewall in the middle. A firewall is a subset of a router; they're built to do NAT so IP addresses don't carry across properly.

The setup in the linked explanation and diagram uses dd-wrt firmware to turn the firewall into a full router, allowing IP addresses to flow both ways without any address translation. I don't think the asus firmware can do that, which is giving you the one-way trouble.

Add a full router (or use a computer) to route between the subnets, and set static routes on each Asus box to that router.

-jk