Default [Q] no 3g on samsung galaxy express

i have a samsung galaxy express i437p AT&T. i got it unlocked and using a different sim card besides AT&T's. the problem is, i never get 3G. its only stuck in 2G. i was thinking it has to do with my location but then i have moved around to different locations thinking it will switch to 3G but still same old 2G. Also the part in the SETTINGS that allows me to switch network mode is blank. in other words i don't get the option of switching. could it be AT&T have locked down the network mode? i need help with this.