Default Galaxy S4 9500 Hotspot problem?

I bought my GS4 outright and I am on prepaid (Vinaphone - Vietnam). Today I tried portable hotspot and everything looked
fine and my lpad connects but the internet just doesn't work. My 3G mobile data works on my S4 but hotspot wont
work. I tried my laptop and my PC so its not the Ipad. My friend had the GS3 and that worked fine. My GS4 in 4.3 rooted and stock rom. Any help appreciated! Thank you.