How to force 2G/3G/4G?
Try *#*#4636#*#* in your dial pad (type the * and # also). Under Phone information you will find Preferred network type. Select GSM  or WCDMA from there.

You will find 


WCDMA preferred this will use both 2g and 3g whichever has the
better reception
WCDMA only this will use only 3g
GSM only this will use only 2g