internet

i use 4S in Myanmar, due to my country poor network connection i can use internet only with EDGE. If the phone connect to 3G function then my phone cannot access the internet. Problem is i use at my work and it shows E sign and can access the internet well but when i go back home then it shows 3G sign and cannot access the internet. At my work the phone signal showed 2 or 3 bar and at my home the phone signal showed 1-3 bar. Not so different in the signal situation so i guess just only the EDGE and 3G system. Could you please tell me how to change 3G to EDGE in 4S. I just open the cellular network on then EDGE or 3G is automatically choosing by I phone system i think. I can't find the EDGE or 3G button in 4S too. Thanks.