Network Card Running at only 10mps

I have two gigabit 10/100/1000 NIC’s installed in a peer to peer network computer. This computer connects to a wireless broadband modem, and the other computers on my network connect to this computer for internet access. Even though the NIC’s are identical, the one connected to the modem runs at 10mps while the one connected to my home network hub is running at 100mps. Any idea why I can’t get 100 from the card connected to the modem? I have tried setting the faster speed as the default for the card, then set it to “auto” it still runs at only 10mps. Thanks