I'm not sure if this will work, because I am not set up to test it (don't use PPPOE at work, don't have AppleTalk network at home):
Go to the Network pane of System Preferences. On the "show" menu, select "Active Network Ports". Click the new button, type in a name (e.g. "AppleTalk"), and select the ethernet port and click . Drag it to make it second in the list, and make sure it is checked.
Now, switch the "show" menu to "Appletalk" (the one we just created). Leave the TCP/IP tab as-is, make sure PPPOE is unchecked on the PPPOE tab, and make sure "Make AppleTalk Active" is checked on the Appletalk tab. Click "Apply now", and test your printer.
This shouldn't effect your existing PPPOE setup at all, but if you find it does you can always delete the "AppleTalk" port that we just created in the "Active Network Ports" screen to revert back to the way things were.
Reply back with the results.