explains the process
However the ZTE hotspot supports GSM1900 and 4G Band IV, neigher one is used in Brazil so even if the device is unlocked it will not work outside America *USA and Canada*
Sorry it is not better news but at least you know
Best buy was partially correct, out of all the operators in Brazil only one supports 1900
The ZTE will support GSM1900 and this will provide EDGE speeds.
The only operator that supports 1900 is www.vivo.com.br
You will need to go to their site to confirm if the area you will be using it in supports EDGE
The 3G/4G band on the device is Band IV , sometimes listed as 1700/2100 (must have both not just 1700 or not just 2100 as those are completely different things).
The T-Mobile Sonic supports 3G bands I, II, IV and V