Network Unlock/update for AT&T Samsung Note 3 (SM-N900A) topic

Hello,
I got an AT&T Samsung note 3 (SM-N900A) from USA and i want to use it outside Europe and US, the Note have the following specification:
Model number: SAMSUNG-SM-N900A
Android Version: 4.4.2
Baseband Version: N900AUCUCMLG
build number: KOT49H.N900AUCUCMLG
Version:
AP: N900AUCUCMLG
CP: N900AUCUCMLG
CSC: N900AATTCMLG

I successfully root it by using Geohot's Towelroot app :) . but when I tried the Chainfire's RegionLock Away v1.3 app it didn't work for me ( since I no longer have the AT&T SIM card) :( .
I also tried to use the service mode ( *#197328640# ) method but I couldn't find phone control screen :( .
so I tried to Install Custom ROM (build number: Dynamic Kat 2.0) using Safestrap, it worked fine for an android system but after repeating the above methods didn't seem to unlock it :( .
the Note seems to read the new SIM network but can't register on it or read its internal saved contacts :eek:
So can I update the modem of this device to an international modem without bricking it? and how to do so in a Safe way?
is their a other way to SIM unlock it ?

Excuse my little knowledge, I'm new in the android world :confused:

I really need help guys, any suggestion will be helpful, thanks alot.