AT&T do not allow their own firmware to be updated by Samsung and therefore, if you have the AT&T model, G900A, you will need to be registered on their network to receive updates. See,
#1.6 of
42 Galaxy S5 models - Dummies Guide for more details.
To make matters even worse, AT&T do not make their firmware available through the usual Samsung sources, so that you could flash it yourself. However, a very nice guy, over on the xda developers forum, called
morphfxf has uploaded a flashable copy of the AT&T firmware for Marshmallow, Android 6.0.1, as well as full instructions on how to flash it. See...
Galaxy S5 Android
version 6.0.1, Marshmallow (G900AUCU4DPH4) release