Just a thought, have you tried changing the wifi channel? Your router is dual band. Have you set up your router to show the two different bands? Some devices have a better wifi signal on a specific band setting. This article may help. This is certainly true with my own Brightbox router.
click here came across a similar situation on the BT community forum where the advice was to switch off Smart Set Up and Smart Wireless.
click here you thought about asking BT for their latest router?