Your description of the cabling is not clear but it must surely be from the UNI-D1 to Internet on the Dlink.
The configuration description sounds simple.
Does the Dlink auto-detect the type of connection or did you make the selection?
Your only options seem to be "DHCP Connection" or "Username/Password Connection". The latter seems more likely.
In the manual (p50 Configuration IPv4-PPPoE) has extra settings.
The Dlink's System Log might have some info when it tries to connect.