How to turn off mobile-data/3G for each SIM card separately in Samsung Galaxy Mega Duos?
I had the same issue. Found this solution on another site:


Open "Settings" App
Go to "SIM Card Manager"
Tap on "Data Service Network"
Choose the SIM as the Data SIM


Note that only the first SIM slot supports 3G Internet. So if you want to use 3G you must plug your 3G SIM in the first slot.