No worries. It does seem illogical. This page has some explanation a little ways down.

Tomato and DD-WRT also have ways to turn the WAN port into an additional LAN port, but that's not really necessary.

Not to beat a dead horse, but let me put in one more endorsement for Unifi, with a pro/con list

Pro:

- seamless roaming between access points, which could be important now that you're going to have an AP in the lower floor that really could deliver signal to the first

- POE means you don't have to run power through that hole

- a real, included mounting bracket will make the result look great, like just another smoke detector (you'll see no wires)

- you don't have to worry about all this configuration confusion (don't have to deal with DHCP or flashing firmware, and there's only one place to plug any of the cables into

Cons:

- more expensive, requiring two APs and a router (you'd disable the wifi in the router). The base Unifi AP costs about $70. The longer range version, which might not be necessary in your configuration, is about $83. Lastly, they have a brand new 802.11ac dual band version that's about 40% of the previous cost at $99.

- the management software can be a little annoying at times, but they've really improved it in recent months with a big visual refresh


In the end, I totally understand going the less expensive route, though.