I just quadruple checked the Telecom website - I can't find anything in their self-help support section that talks about this problem specifically (I had checked their support site multiple times trying to solve the issue myself). They do have an article I wish I'd found that says if your connection is ok but you can't get to websites, check your DNS settings - but nothing specifically about the Technicolor modem and Win 8.1. Nothing on the Technicolor modem support/specs pages etc. Even more annoying they have an article that suggests DNS might be the problem with the symptoms I was having, but again none of their techs even suggested checking the DNS let alone manually configuring it. The search is so bad it's hard to be sure, but I didn't see anything and it certainly didn't come up in any of the promoted support links they put everywhere for common problems.
Unfortunately I have to go out for the day shortly so I don't have time to sit on hold to get through to Telecom and discuss the issue with them again, but I will do as soon as I have time. Hopefully I can get them to document the problem somewhere others will find it.