*Post has been modified to exclude use of 192.169.x.x ip since it isn't convention to use it as a private ip. You should not have to use cascaded router unless you have bought a public IP space which is why you will get the error involving WAN wide subnet when you try to enter in 192.168.x.x with a /24 subnet into the cascaded router boxes.*
Information is a bit scattered around, but unless you are a network engineer (which I am not) you are not going to have a fun time trying to configure a cascaded router. The idea is better drawn out as having a gateway (ATT) passing traffic to the router (your router with DD-WRT) and your router being the point where most of your home network devices will connect to.
On the gateway (ATT Fiber router), you will likely have a 192.168.1.254 address. This is the address you will use for accessing the setup. After accessing setup, you want to follow these steps to turn on IP Passthrough from Linksys https://www.linksys.com/us/support-article?articleNum=293959. It is really easy as long as you have the WAN Mac Address. Shut off all of the other settings under firewall (Packet Filter, NAT Default Server, Firewall Advanced).
You should be able to see a WAN IP Address assigned to your router (DD-WRT).
Next, configure your router's local IP address to be 192.168.2.1 / 24 on DD-WRT's setup tab as this will be the router's ip and set DHCP to start from 192.168.2.2 and DHCP will assign ip's from this range.
Save and apply settings and it should be done on the gateway side.