If have Uverse, either adsl2+ using Motorola 510 or VDSL using 2wire/pace 3600, 3800, 3801, 3812, 5031 or Motorola 589 then the authorized RG must be first to outside connection, all other consumer personal equipment (CPE) is placed behind in either DMZ mode or IP pass through.
Not familiar with legacy ADSL products (non Uverse) to comment.
*I am an AT&T employee and the postings on this site are my own and don't necessarily represent AT&T's position, strategies or opinions.
Since Graduating Trade School for Computer IT, I have tinkered with Network Equipment since to today. One device IS an AT&T DSL Modem Gateway. If You log in to your router's IP Address, usually 192.168.1.254 or 192.168.1.1 (check your PC's default gateway if neither work), there is a setting page for "Home Network". Once there, go to the sub-page "Advanced". Once In, Setup your Router's IP Address in the same range as your ASUS LAN DHCP. I.E if your ASUS is 192.168.1.x (x=DHCP), set your IP to any range from 2 to 253. To ensure there is no conflict with other networked computers, I suggest 225 to 250 (the rarely used leases). After You Restart the AT&T, connect the LANs of BOTH Gateways, so the Wi-Fi Access Point is not Firewalled. In Experience, Configuring AT&T Firewall Settings is a pain, especially when specifying ports you want open or closed........................the point is: ignore the firewall, since it only filters the Broadband/DSL Modem Built-In.