A common issue encountered by NTL users is being unable to browse the web after cloning the MAC address usually used to connect and assigning it to their router.
A quote from a member of NTL tech support offers a solution
Hi All
If it helps at all the MTU packet size is set to 1500. As to the matter of clone mac addresses although it is possible to do this the reason that u cant browse on a STB after connecting a new device is because the stb requires a short registration for the mac address of the new device. If u check that the wan address is 10.x.x.x then open your browser and type “start.ntl” you will require your pid and password.
If anyone has any question regarding ntl specifically i work for their tech support so ill be happy to answer but please dont all ask at the same time 🙂
Cheers
Niteblind