Are you sure this is router and not an ADSL modem/router? ( a model number would help us google). If so, there shouls be a port labelled 'WAN' or similar, which should be connected to the STB, and other ports (possibly labelled 'LAN') which should be connected to the PCs. The symptoms sound like you may have the STB and PCs all connected to LAN ports, so the 'router' is acting as a simple hub.