DNS/DHCP Server and Cascading Router

Hello, 

This is a general discussion of my experience with DNS and DHCP with my own router and a question about cascading.

I had been on the bonded pair 45Mbps internet with ATT for years, and I use a TP-Link with DDWRT software as my main network router for Ethernet and WiFi, using a LAN port on the ATT modem to the WAN port of my router.  This has worked quite well for many years. It allowed me to monitor network traffic, control which DNS I want to use, and prevent me from going over the 1TB data cap.

I recently switched to Fiber and have the BGW320-500 gateway.  I no longer have the data caps to worry about, but have noticed that the WiFi is much better on the ATT gateway, and my router has become a choke point for speed on the WAN interface.  The main reason I want to keep using my router is so that I can control which DNS server to use, which we all know there is no possibility in having that control with ATT hardware.

What I decided to do is keep the WiFi on, but turn off the DHCP on the ATT gateway.  I then connected my router LAN interface to LAN interface on the ATT router (5GB speed).  My router still handles all DHCP and DNS requests, as well as some Ad Block plug-in which is great.  

This is all working fine. But one thing I am curious about is the Cascaded Router function on the ATT box. Right now it is turned off, and I really don't see a reason to tell it that this router is behind it.  The DHCP requests still work fine, the request will come in for wired or wireless devices, either on my router's LAN or WiFi, or the ATT gateway LAN or WiFI, and my server will answer and hand out the IP.  It now becomes the gateway for all traffic.  What good would it do to configure the ATT gateway to tell that this router is a cascaded router? What extra would it get me? To me it is still serving the same purpose, albeit on the same subnet.  I can see how cascading it to the WAN port could then give me more privacy for an additional network, but like I mentioned, the WAN port is now a choke point and limits the bandwidth.