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). The gateway, link between them and the router form a cascaded router.
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 126.96.36.199 on DD-WRT's setup tab as this will be the router's ip and DHCP will assign ip's from this range.
Back on the gateway device setup, under Home Network go to Subnets & DHCP.
* Turn on Cascaded Router.
* Set Router Address to 0.0.0.0.
* Set Network Address to 188.8.131.52
* Set or Leave Subnet to 255.255.255.248
Save and it should be done on the gateway side.