|Had to think about this one. |
So, on your DDWRT set your WAN to a static IP address in range of your R1 and turn off DHCP on your DDWRT. Now get two Ethernet cables and connect one to the WAN and one to a LAN connection on your DDWRT, connect both of those to the LAN side of your R1. Or you should be able to connect from your WAN to an empty LAN on the DDWRT, but let’s not complicate this right now.
There is a way to bridge the connections internally with the DDWRT, but I can’t remember now, you can play with that later.
Now on your R1, just for a temporary test, lets DMZ the IP address that you set up on your DDWRT and see if your VPN is working correctly. If it is remove the DMZ and set up the port forwarding of the VPN port to your static IP address.
I have a similar setup at my shop.
Let me know how it goes.