I'm a little confused. You say that the ATA would not pass the internet on to the connected computer when set up in PPPoE mode, correct? You were able to regain access by doing a factory reset to the ATA, and when you re-entered PPPoE settings, you lost access via the LAN port, right?

What happens when you save the PPPoE settings? Is your PC's IP address lease forcibly expired? (Or does the adapter reboot, and then not assign an IP address to the PC?)