SSH through standard Belkin router to Asus Tomato router

view story

http://serverfault.com – Disclaimer: My understanding of networking concepts is weak; I only get the basics (probably why I've ended up here). I've set up SSH on the Tomato firmware on an Asus N10, via port 22 with key authentication. I've tested the keys by connecting with putty directly to the router when connected to its network. That works OK. But this router is behind a Belkin (F5D7632-4) router which also acts as modem and when I try to connect through with the (dynamic) public IP it times out. I'm guessing it's something to do with the NAT? My putty settings are taken from various online tutorials, but it's (HowTos)