If I understand you correctly the wireless (WAN) stopped working so I presume you are connected to the router via ethernet cable because you say your internet connection is fine. Or do you mean the ADSL LED is showing connected?
You also say you reloaded Windows - at which point? My initial thoughts are: did you hide the SSID in the settings - some routers can do this so they can`t be "seen" but I would have thought you would be able to get at the admin page yourself. Secondly, It`s 4 months old - take it back and get a replacement! If that one doesn`t work you have got problems!