If you used a cross-over cable to your PC then you will probably need a straight-thru one to the router.
Also, your STB will only recognise one MAC address. But your router will have a different one to your PC. See this FAQs page:
http://homepage.ntlworld.com/robin.d...tips/swap.html
You may be able to register your router by powering off and on the STB and then try registering via the router (see the FAQs page again). If this doesn't work you should be able to get the router to 'clone' your PCs address.