3G not working properly on iPhone 5s (ver 8.3)

a bit of background info: I bought my iPhone in Canada, a bit over a year ago. I'm currently living in Brazil if that matters

anyway i got a 3G plan not too long ago and it works but only when it wants to, and it's incredibly slow. I thought that was normal considering Brazil isn't exactly known for its amazingly fast 3G speed. that was, until yesterday when I connected to my dads hotspot, and get this: he's got the same plan as mine, his phone was also bought in Canada (it's not an iPhone but still) and it was FAST. Like, really fast. Faster than my 3G ever was.

ive tried everything to make it faster, from removing my SIM card to resetting the network settings to restarting my iPhone, everything. this is making me SO angry! the only thing I haven't done yet is restore the whole phone to its factory setting, which I don't want to do unless it's extremely necessary. Please help!!

P.s: sorry about my English :/ being Brazilian + mad = not the best combo out there