The IPTV stuff may rely on certain QoS settings. The TV QoS part of the Asus router may be the place to fiddle but without any indication of the settings from TalkTalk you are playing a long guessing game.
In terms of performance, I found the router worked fine on both a BT WBC DSLAM and a Sky one, but it is possible that the way TalkTalk have configured their hardware at the exchange that the Asus does not like it as much.
Your SNR down of 10dB is costing you some speed though this link illustrates this
http://www.coolwebhome.co.uk/calc/index.php?param=RG...
The range of speeds to expect is 3300Kbps to 5600Kbps
With a 6dB target noise margin you should get around 4491 Kbps