Constantly loosing 3G connection

I constantly loose 3G connection for edge in my Iphone 5S, especially in my office, at one of the commercial centers of my city (oil & gas). When I contacted my carrier (TIGO), they told me that 3G signal (is not strong enough or) has interference from other devices and that all Iphones grab the strongest signal around and cannot be set to use 3G only (Android phones do).

Don´t know how important Third World´s Iphone market is for Apple, but our 3G networks are not mature yet (no LTE at all). So please add this feature in a future IOS update. All Third World´s Iphone users will be very thankful.