The problem is almost certainly with the "wireless" connection between your computer and the router and not with the router's connection to the ISP.
While connected by ethernet cable, *NOT WIRELESSLY*, go into the router's wireless settings and change the Channel number it broadcasts on. Most routers use "6" or "11" by default so try one of these or, if already using one, try the other. Avoid channels 1 & 2, which use frequencies shared with many other wireless devices such as cordless phones, and avoid 12 & 13 which many wireless adapters won't connect to, because they are not legal in certain [non-UK] countries.
Also, remove any existing wireless 'profiles' in the PC's settings click here and then go through the procedure to "Connect" again.