This might work :
Open the dialer and give in this : *#*#4636#*#*
Select phone information.
In the list that comes up is the SET PREFERED NETWORK TYPE option.
Choose that one and test another option ( there is quite a list
I've used it to test on having 2g only , 3g pref. and 3g only
( a late miui GB only gave me 2g and 3g pref. with 3g not working ... )