With Plusnet and BT you get supplied with a EchoLife HG612 (FTTC VDSL NTE) Modem which will work with their kit automatically, but if you want to use your TP-Link TD-W8960N wifi router instead you need to configure it to be a PPPoE router which can be tricky if you dont know where to click.
ticket.tp-link.com/index.php?/Knowledgebase/Article/View/272...
If you have a cable modem broadband service and you still want to use TD-W8960N as a pure wireless router, you can follow the steps below to configure one of the LAN ports working as the WAN port (We call it "EWAN"---Ethernet WAN).
www.tp-link.com/en/faq-541.html
How to turn TD-W8968V3V4, TD-W8950N,TD-W8950NDV2,TD-W8960N V4-V6 into an NBN-ready wireless router ?
www.tp-link.com/Resources/document/TD-W8960N_V3_User_Guide.p...
TP-Link TD-W8960N V3 Wifi Router User Guide in PDF format.