If the ISP WiFi does not provide enough coverage and someone purchases a mesh WiFi system, they almost always elect to disable the WiFi part of the ISP device so that it will not compete with the mech system. There are not many WiFi channels available and neighborhoods are clogged with WiFi networks, so eliminating a WiFi radio blasting inside your own house is a good idea.
As for performance, there is no obvious reason that the Orbi would show slower WiFi performance than the ATT WiFi. Generally speaking, having "two routers" is also not a great configuration, as Netgear explains in this article:
https://kb.netgear.com/30186/What-is-Double-NAT
I have no personal experience with ATT, but my observation is that Orbi users frequently elect to put the Orbi into Access Point mode in order to avoid the "Double NAT". (On the Orbi web interface, Advanced Tab, Advanced Setup, Router/AP mode)
You might want to double check the Orbi-to-ATT conntection rate. On the Orbi web interface, click on the Advanced Tab, then click on the blue box "Show Statistics". The WAN link should show as 1000M/Full
