Have you checked the APN settings for your network?
You can do this by going to Settings > More > Mobile Networks > Access Point Names, select your SIM (if you have more than 1 inserted) and then check the settings with your network operator.
Also, go to Settings > More > Mobile Networks > 3G Service and ensure the 3G service is enabled on your active SIM
Also in Settings > More > Mobile Networks > Data Connection and make sure this is enabled for your active SIM