I had a somewhat similar problem/experience with adding a 3rd party router to my AT&T's modem/router. For an AT&T U-VERSE modem/router it is impossible to shut off the DHCP. (that is just how AT&T like to corner the market), however, you can use the 2nd router (Cisco) as an access point by going to Cisco router configuration and making the LAN IP address in the same scheme as your router or modem. For example:
Router 1: 192.168.1.1
Router 2: (by defualt) 192.168.2.1 ~ Change the ip to 192.168.1.2 or 192.168.1.253/254
P.S. If you have a standalone modem, it is possible to just shut off the DHCP of the first modem and let the Cisco router take over .