Found your old thread just for perspective.
I get the idea that you want to mimic to something that worked earlier as in the older "Chaos Chalmer" release, but do you atleast have a git repository with the changes you made earlier?
Do you have real seperate ethernet interfaces in your device for WAN and LAN, or a builtin switch with just vlans to seperate ports? Does the WAN configuration work on the old firmware and whats the output of swconfig in the "old firmware" (if the device uses a switch) the same as "your own custom firmware"?