AX1800 setup. Will not connect to AT&T router.

What att router? 

what router did you buy? AX1800 is a speed

Also, att tends to supply modem/router combo device. 

Cascading routers can cause issues as it causes a double nat. 

https://kb.netgear.com/30186/What-is-Double-NAT

some att devices you can put in passthrough/modem only mode. 

Others you need to put the netgear's ip address into the att devices dmz. 

Or you can take the easy way out and put the netgear in access point mode. It does disable some features on the netgear. https://kb.netgear.com/26765/Which-features-are-disabled-on-my-NETGEAR-router-when-it-is-set-to-AP-Mode