3g/4g problems from korean variants

Hi im kinda newbie here..im from philippines.i just want to ask why i cant use mobile data from korean variants in my country..the 3g/4g is enabled and apn is set already.on the top of my phone it has a 3g signal but when i tried to do surfing,you tube etc etc.it always says no internet connections.i also have lg g2 d800 and mobile data works well.why is that??i have 4 korean variants devices and they all have issues.pls help me on this matter.