Unlocked iPhone 6 won't work with multiple SIMs

[SHNXX]

I bought my unlocked iPhone 6 in U.S. and have been using it in multiple countries.
For some inexplicable reason, I cannot use it in Korea right now and I've tested three different SIMs none of which works.

All of them can connect to the network, make calls, etc and it says 3G but the internet doesn't work.
Internet works when I'm on Wifi.
Just to make sure, I had another person try out the SIM that doesn't work for me and it works fine for him.

What gives?
I've tried resetting network settings multiple times.

 

[SHNXX]

Does this have anything to do with APN?

 

[SHNXX]

Ah this seems to be an odd quirk or a bug in the ios8. Resetting doesn't actually delete all APN conflicts so a previous APN can prevent the installation of the one that you need. Problem solved.