Bad iOS/Apple Connectivity (solved) and Slow Speeds - Orbi RBR40 - FW v 1.10.1.2

I would guess that if you replace your ATT router with a simple cable modem, and use the primary Orbi as your router (as it is designed to be), you would get better performance.   It is undoubtedly more complicated to get the settings right with 2 routers trying to do what they do.  We have our coax cable line in to an Arris SB 6190 cable modem with ethernet cable direct to the Orbi router, and are getting very consistent performance all over, with all default settings on the Orbi.