I was trying to establish a PPPoE connection between a 851W and a Siemens SpeedStream 5100 ADSL modem with a dynamic IP, and while I was able to enter the details, the 851 wasn't able to get an IP from AT&T but if I set PPP on the modem and setup the 851 like I would a Linksys(or anyother home router) on Comcast cable, it works fine via DHCP. What is going on?

I am using SDM to configure the device, not CLI.