Bungie recommends using UPnP to automatically generate the port forwarding rules rather than entering them in yourself. I have tried UPnP with both routers, so I doubt it's defective hardware, and the Xbox reports Open NAT with both routers. That being said, I can certainly try static port forwarding just to see what happens, it will just be a bit of effort to reconfigure my console + router.

I was able to try out port forwarding with a static IP but I am still experiencing the code weasel disconnects.

Have you used DMZ?