Best mate used that draytek model listed and had exactly the same issue of lost speed when plugged in. Before you ask he does work in the field as a PC/Server engineer and is MCSE certified to the hilt, has been doing installs for twenty years now and he never did suss it out. He just plugged the draytek into the modem. And yet local PC shop is using one and they have no issues. Totally mad!
I never did ask him or bother to look, and it would be a complete guess its still connected thru the modem port on draytek, and the settings are gonna be PPOE. ray_dorset most likely knows the answer to this as he seems a really knowledgable guy
in the meantime look here: