I found this out the hard way and Samsung's support is useless when trying to explain the issue !!
After hours of running around they offered to have it flashed back to stock 4.0.4 ICS but I have to send it in to Plano, TX.
If you upgrade to 4.1.2 either by OTA or Samsung's website the APN Settings under Wireless & networks no longer is available. Also no root has been discovered for this device either so there is no going in and changing APN settings.
** If there is a known working root then you can upgrade and not have any issues. APN settings can be pulled by adb and pushed at that point. But until then you have a read only file system. If you have already done so then you can pull the apn.xml file and see that it contains the wrong APN settings.
*** SAMSUNGS LIVE CHAT SUPPORT IS USELESS