Try a factory restore. If you would not like all your data deleted, then you can root and flash a rom (well worth it ) you can also stop by the att store they may just know what tovdo. When i had that problem i just changed my rom (the software) and it worked perfectly after.

 

There are multiple free apps in the marked called "wifi fix" or "wifi fixer". I used to have problems, but the app actually worked to fix it. I think the one I used was FXR or something? But I found a different one for my tablet that worked too.