I had the similar problem. What you have is a BUG in Nokia. What I did was I took all the settings for the Internet, GPRS, WAP, etc, because I will not use it when I misplaced the capability to choose which AP to utilize. So what I did was to make an environment:
Settings > Configuration > Personal settings > Add > Web
Then go to the main menu> Internet> Web Settings> Configuration> Select the personal values you have created. Now, attempt to access the Internet and have to choose, choose Wi-Fi.
Then return to your personal settings and eliminate the settings you shaped. From now on, you are asked to use each time you access the Internet.
To reset you’re GPRS, can only ask the provider of your network and send it to your phone.