I see wifi networks but does not connect

My noteBook is accessing the wifi networks but does not connect.
See the result below:

asus:/home/sergio # /usr/sbin/iwconfig
lo        no wireless extensions.

wlan0     IEEE 802.11bgn  ESSID:off/any  
          Mode:Managed  Access Point: Not-Associated   Tx-Power=20 dBm   
          Retry short limit:7   RTS thr:off   Fragment thr:off
          Encryption key:off
          Power Management:off
eth0      no wireless extensions.

I do not know what to do with this information.
Please, could you help?

I detected something strange: The notebook can connect with the wi fi of my cell phone
Is it possible for a noteBook to connect to a router from a mobile phone and can not connect to the router of a modem?
I have another macchines like a netbook, two cellphones and a PC and they all connect to the modem without any problems.
So I think the fault is not from the modem
Please, any idea?

That can depend on the WPA/WPA2 implementation on the router itself. Some were faulty. Mixed mode did not work, had to drop back to only WPA, some to WEP, not a good security solution.

If that is the case, you are better off getting a newer modem/router, or have the modem/router set to bridge mode (just pass-through) with wifi disabled and run a better 2nd router off of it.

fwiw (observation)

if signal strength is shown as below 60% a reliable wifi connection cannot be established

if below 50% its impossible

an HP-Pavilion laptop internal wifi does not work in some home locations. When a usb Logilink WLD151 is plugged in, there is never a problem