That's annoying you can't get the most out of the TV box.
It would be good to know if your current connection is as good as it can get or if there's room for improvement. If you can post the connection statistics from the router that would give us a good idea. By "connection statistics" I mean the kind of figures in the second post here. I don't have that router so I'm not sure how you get the stats from it, sorry.
If you can't get those stats then you could go to the BT exchange checker here and click on "address checker". You can't use the phone number checker as you don't have a BT number. That will give you a good indication of what speed you can expect, the "Downstream" speed for ADSL 2+.
The FTTC lines will indicate if fibre is available.
An OCE (online customer executive) from TalkTalk will be able to give you a better answer as they can look at your line directly, they'll respond usually in a day or two, but meanwhile that may be of some use.
Good luck!
John.