[Q] Network unlocking my I9500 and knox topic

Hi guys, I wonder if I can get some help unlocking my I9500. I'm traveling to US next week and would be acquiring a t-mobile or at&t prepaid sim to use the the phone, so I need to unlock it.

First of all, I have verified that my phone is locked to accept sims only from my carrier.
Second, yes, I have tried the methods explained http://forum.xda-developers.com/show....php?t=2282683 , but I always get:

MENU NOT EXIST
PRESS BACK KEY
Current Command is 111

If relevant, here is some info from my s4:
android 4.3(official from my carrier)
baseband: I9500BUEMK1
build number: JSS15J.I9500UBUEMK1
AP: I9500UBUEMK1
CP: I9500UBUEMK1
CSC: I9500UWMEMK1
RF Cal: 2013.4.24
HW Rev: MP 0.800

I don't want you any method that may void my warranty or alters the knox flag.

Are there others "not invasive" methods to network unlock my 9500?

If I have to get a network unlock code from some site in the internet, which ones do you recommend that are reliable? would using a unlock code alter my knox flag?

Thank you so much in advance.