[Q] Can't change band frequency. Why? topic

I'm from Brazil and 3G signal from my carrier is really poor at my home.
So my phone keeps switching between frequencys every time and it causes instability on my signal.
I'd like to change my band to use only 850Mhz like I did in my old Samsung device in order to solve my problem, but when I type *#*#4636*#*# and go to Phone Info > Menu Button > Radio Band Selection there's no options to pick. Why? How to solve this?