Hi,
You have to reboot the cable box (power off for for 30 secs then restart) or 'clone' the mac address of the network adapter you were using to get it to talk to the router.
I don't have an STB but you used to have to reregister a changed netword card by going to
http://start.ntl/
but I think I saw someone saying the procedure has changed....
I'm impressed you got the USB to elthernet device to work under Linux - would have thought it would be much easier/ better to use a real network card!