The SpeedStream 5100b/4100/4100b modems have integrated PPPoE client software and a corresponding Internet light on the front panel to indicate whether your modem has established a PPPoE connection to AT&T Yahoo HSI, when the modem is configured to initiate the PPPoE session.
do you get redirected to an AT&T support page that tells you that the modem password does not match the PPPoE account password? One way to fix the problem is to login into the router and goto setup => basic setting and hit the "test" button. The test always passes and the internet connection is restored after that.
192.168.1.1 default login is
It may be something like you need to lower the router MTU, especially if you have a DSL connection. Try values of 1492, 1400, 1350 as a quick test. You could also try disabling the router SPI function (sometimes called Port Scan and DoS Protection).
PPPoe stands for point to point protocol over ethernet. With PPPoe you more than likely have to enter a username and password to establish a conection with the DSL server at the main office. While if you are using a connect that is just you Local Area Connection you are simply connecting accross your LAN with no need to contact the server first. So using the LAN is always on while the PPPoe requires user input to work (it probably also times out if your not using it continuously)
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