Unfortunately, the AT&T and Verizon firmware updates are released by their respective carriers, direct to their customers. This is unlike all other models, whose firmware is updated via Samsung. Therefore, information and downloads are not available from the usual Samsung sources.
Since it usually takes AT&T and Verizon around 8 weeks to modify and release a new build I do not anticipate, as per the S4, S5 and S6, regular updates in line with the Samsung released Global firmware releases. They are currently receiving the PD1 and PD3 updates for April 2016. The U.K. unbranded model is now on its 3rd firmware release since launch.
I should have just bought an International version since I paid in full for this device. I just wanted it at release and was unsure if how long it would be before unlocked versions would be available, sigh.