Ok, when you go back to your router page, try setup, then "wireless settings" then click the button manual wireless network setup. From there you will find "channel width". What is it set to? please try 20mhz. Also, try changing channels to see if that helps.
Also, have you tried other security such as WPA? Or have you tried without security?
One other thing, you should look into updating to the most recent firmware for the product. You will find the current version on the top right corner of your d-link page.