I had e-smith working with NTL broadband without any bother, but it was in the days of the old dial up customer support, get put on hold for 2 hours and register manually.
You mention cable STB, are you plugging directly into the TV set top box? or do you mean the cable modem? I was under the impression you had to have a cable modem and you couldn't use the ethernet port on the TV STB.
If it is a Cable modem then here are a couple of things to consider:
1. You must register the MAC of the cable modem, NOT the ethernet card in your e-smith server.
2. Make sure you external interface IS really your external interface, I've seen a few problems related to having them the wrong way around.
3. If NThell's automated registration service is like their other registration servers then it's probably using HTTPS:// not HTTP:// which lynx doesn't support.
Your best bet would be to phone and get NTL to register the MAC address for you