No signal gain after using wifi antenna

Hello.

I have bought this [1] wifi antenna (21 dbi) to see and possibly (if open) connect to as many 2.4Ghz 802.11b/g/n networks in a small village (1km radius) as possible. I have used Asus WL-500W router with latest binary trunk Openwrt "ATTITUDE ADJUSTMENT (Bleeding Edge, r32647)" and replaced the middle antenna with the abovementioned connected with 5 metres cable.

The thing is, when I place the antenna to the roof and connect it to the router, there is no signal gain, no more network shows up. Sometimes it even worsens the signal. I used airodump's PWR column values to measure the signal. I noted also that the PWR values are all negative numbers, usually from -69 to -90, sometimes -1. This is my screen [2].

Am I missing something? Do I need more/another hardware to catch the wifis?
tx

[1] tinyurl D0T com DASH myAntenna
[2] tinyurl D0T com DASH 9aaby7p
P.S. Sorry but I cannot insert URLs as a newbie.

signal strengths are indeed negative numbers. lower absolute value - higher signal strength. -50 is 20 db hotter than -70

Yep, I have worked that out already.

Anyway, I am curious why the antenna does not work, because putting my router outside shows much more wifis while conncecting antenna dont. The router has 3 antennas, maybe OpenWRT should be set to use only the new one. Or should I add some kind of amplifiing device?

What coax are you using? At least LMR-400 or equivalent?