You can't set the media type on a PPPoE interface (or any type of virtual interface) you have to set it on the parent interface. If you don't have that available because it isn't assigned just add it as an new interface and set it as type 'none'. Then set the media type there.
What is the actual hardware the WAN is connected to? Are you sure it's not forced to 100Mbps, have you tried connecting to it with something else?
Steve