I found this awhile back at DSL Reports when trying to solve this same issue with my Dad's setup.
1. Connect your computer directly to the modem (bypass the router), you may have to reboot.
2. In your web browser browse to »192.168.0.1 This address will be located on a yellow sticker on the bottom of your modem.
3. The first screen will ask you for your Modem Access Code. This also will be located on the same yellow sticker.
4. Make sure the modem is setup to perform the PPPoE login.
5. At the login screen type in your SBC member ID and password.
6. Click Connect.
7. If everything is succesfull you will get a screen that says, "You Are Connected." and your Internet light will go solid green. If you username or password is invalid you will get a screen stating such. If there are other problems you will get a screen stating, "The DSL connection is down."
8. Plug the router in between the modem and your computer.
9. If your router has an IP address of 192.168.0.1 (the D-Link does) then you should change it to something else like 192.168.2.1. 192.168.1.1 will work, but if it assigns 192.168.1.64 to a device, it won't route correctly, since the 5100b will assign the router 192.168.1.64 on its WAN side.
10. Setup your router for DHCP on the WAN side, or set it statically to 192.168.1.64 with a subnet mask of 255.255.0.0
11. You should now be online, given the possible need to restart your devices.