m0n0wall Forum - use shared internet connection on UMTS laptop as WAN till ISP is done

Following situation: new ISP after move, bad choice, odyssey goes on.
Got a couple of machines behind a m0n0wall (can't check version right now here at work, think 1.2.xy).

And a laptop with xubuntu 10.04 and a UMTS card.

Now until the manky ISP (Congstar Germany btw) gathers the wits to finally provide me with DSL I'd like to "make" that laptop's RJ45 the gateway for the m0n0wall's WAN interface.
The machine has already internet connection sharing set up ( pretty much followed there instructions, the easy part with network-manager: https://help.ubuntu.com/community/Internet/ConnectionSharing ) and listens on 10.42.43.1 for dhcp. Another ubuntu machine wired there perfectly has internet access. To be honest, I'm not quite sure how that ICS works en detail.

Now - how would I make the m0n0wall provide internet on LAN from UMTS via that laptop?