Following in the footsteps of T-Mobile's and Verizon's versions, AT&T's Samsung Galaxy S8 and S8+ have started to receive software updates which enable Daydream VR functionality. These updates also bring with them the August security patches, making the S8 duo on AT&T current from that standpoint.
That's not all though. Alongside the usual "stability updates" and "various other updates and improvements" (all unnamed as you'd expect), the phones are getting network performance enhancements, Wireless Priority Service, UX updates, navigation bar auto-hiding, and updates to Email, Calendar, and Knox. Advanced Messaging has been updated too, and so has Smart help.
The update is about 490MB in size for both devices, and after applying it you'll be on build NRD90M.G95xUSQU1AQGL. Replace that lowercase "x" with "0" if you have the S8 and "5" if you own an S8+. As usual with these things, expect the full rollout to conclude in a few days, since it's probably a staged affair.