I use NTL 600K broadband for my PC internet connection. I'm nowhere near an expert but here is a suggestion...
Has the ethernet card in your mediator ever been used successfully on the NTL connection before, even if it was with a PC? When I first went online with NTL, I was using a PCI ethernet card, and when I switched to my Abit motherboard with a built-in ethernet port, I couldn't connect. I rang NTL and they told me that for security reasons, an ethernet card had to be registered with them (using it's IP address I think) before it would work on the NTL network. Perhaps this is part of the problem?
On the other hand, this may be PC-specific, or I may be talking complete gibberish!
Steve.