Hello my iPhone 4 does not connect to the internet anymore(wlan still works) but it still shows the 3G symbol. I have that problem since ios 5.1

Ich still shows the 3G symbol but I cant connect to the internet anymore. Since ios 5.1 it disconnected every 30 minutes untill i restarted. De- and reactivating the mobile internet never helped. Now i dont get a connection. It quite not works.

I never had problems untill ios 5.1

of course apn, username and passwort for the carrier are correct.

My carrier is simyo

(im in germany)

Thank you for helping me