Hello!  

Normally, I use a ATT BGW-320 as my home wifi modem/router.  However, our modem is on one end of the house and most of the household is on the opposite end of the house so we've been having some trouble with a stable wireless signal.  We decided to buy a TP-link Archer AX21 router which is physically connected to the ATT gateway via a long RJ-45 cable.  We're able to connect to the router via Wi-Fi and access the internet.  However, our network resources don't seem to show up.  I couldn't access things like my shared NAS or media servers.  I did notice my devices that connected to the TP-Link router seemed to have IP addresses in 192.168.0.* subnet which is not what the ATT gateway offers so I checked my network topology. 

I noticed the network topology of my devices:

I also remember during quick setup, I saw the following message. 

My ATT BGW-320 is definitely a modem/router.  I clicked that option and was presented with the following:

The first option is not feasible for me as the ATT modem is connected to a lot of our network resources such as NAS, Raspberry Pis that are in the distant area of the house.  The second option doesn't seem feasible either because AT&T states they do not offer bridge mode on their modem/routers and disabling the wireless network of the modem/router is inconvenient as we would like Wi-Fi to be strong in that area of the house.  

How can I make my TP-Link Archer AX21 be connected to the ATT BGW-320 without creating its own private network?  I simply want to extend the network from the BGW-320.  I tried turning off the DHCP server on the AX21 but that prevented me from getting an IP Address from the router.  Any ideas how to proceed?