Shaw Arris SBG6782 --> D-LINK DIR-859 --> QNAP NAS
When I enable passthrough on my the SBG6782 so that my internal router (D-LINK DIR-859) gets a public IP address it prevents me from accessing my NAS externally. As soon as I disable pass-through mode on the SBG6782 and rely on specific port forwarding (in both the SBG6782 and DIR-859) it starts working.
Why in the heck? Pass-through is supposed to let all traffic through, which I assume it's doing since my D-Link router then gets a public IP instead of an internal 192.x IP from the SBG6782 router, so why would I no longer be able to access the NAS? It's as if enabling passthrough starts blocking ports. Is there some other feature / setting that gets enabled when pass-through is enabled? Some firewall settings perhaps? Can't find anything in the interface aside from Firewall > Protection Level which is set to off.