I checked on ATT site.  That modem only runs up to 10/100.  So switching to 1000 will not allow the modem to connect.  You either need to get a new modem or change it back to 10/100/1000 auto. Either way...your not gonna be getting over 100Mbps from your DSL.  Usually with DSL you won't get over 12Mbps.  You only really use 1000 for fast file transfers between computers in your home network.  I do this in my network.
I can't connect when I switch my router setting from 10/100/1000 auto to 1000. I have a desktop pc with Intel  82566DC gigabit NIC, I have an ATT Westall Proline modem. I've changed the Link speed setting in the NIC under advance setting from Auto Negotiation to 1000 full duplex. Any advice would be great. Thanks *****************************************************************************  Roger1955 ************************************  
Dir 655 Setting WAN port speed to 1000 mbp