Difference between router and gateway mode in shibby mod

Hello,

I have a cisco DSL router with does natting and I have connected RT N-16 with tomato firmware installed for QOS. The thing is I just dont want to use double NAT instead i want that my cisco does the natting and Tomato does the QOS. Is this setup possible ? Any ideas ?

I have tried to setup it by making the tomato router in router mode but no success. Also I would like to know how the WAN port functions in router mode.

Regards