10Base-T Ethernet (WAN) part of the spec is the issue here on the WAN port this means the router is only capable of a maximum of 10Mbits full duplex.
You need to look out for
100baseTX which is capable of the full 100Mbits - make sure you set the WAN port to the correct speed / duplex though and that it matches on the modem it's self else its all fun and games!
Collisions / Jabber anyone?!