If your ISP allows for more then one device on there network and they handle the routing, you could try this:
Turning a router into an AP.If you try this, you don't need to PF or use anything, it will just be a access point and switch as you'll be connected your LAN cable from the wall out let to the LAN port, NOT Internet or WAN port on the 815. You'll need to set up the routers IP address to 192.168.209.3 I think instead of the .50 in the sticky. Use the same IP addresses you posted in the picture. I thinking that if you can get the 815 in a AP mode and get it working as a pass thru, that maybe you won't have to PF.
Can try this if you want to.