Wifi and data connection problem after jailbreak

HI, everbody.
I'm bought an jailbroken iphone 3g. I can't use wifi and my internet connection of my plan.

Please can you help me ?
My iphone informations :
4.2.1 8C148
06.15.00

Your phone is on the iPad baseband that people used to use to unlock these phones, but doing so causes problems with WiFi and GPS, Bluetooth, etc. You need to downgrade the baseband in your phone. Follow the instructions here:

iClarified - iPhone - How to Downgrade Your iPhone 3GS, 3G Baseband for Unlock (Windows)

Thank you so much for the Answer. I'm gonna try it . Thank you Again