No, I was just aiming for the simplest possible way to set it up, from YOUR point of view. After all, you are the one who has to enter all of the settings, and DD-WRT is anything but "straightforward".

The idea is to factory-reset the WRT54GL, and then just set it up as a straightforward DHCP router, with the WAN port connected to any one of the LAN ports of the upstairs router. Double-NAT, yes, but very easy setup, and slightly better internal security than configuring it as an AP.

I find the DD-WRT interface immensely confusing. Sure, I can navigate it, but.. wow.

A new TP-Link WR720N would be easier to set up than this DD-WRT box, and look nicer too. $20 well spent.