Iphone 5 ios 7.1 unable to connect 3g!! Wht shd i do ?

I just got the Replacement on 15 Mar,2014.

Everything seem to b perfect! Excepted, pit cant use 3g. On the screen, it shows that it already connect to 3g, but it cant use. I already checked my Data from network , they confirmed that My package 3g data is unlimited package. What should i do?