What's the downstream and upstream showing on staus page of W8960N normally? When internet speed drops, what does it show? Try to change DSL mode to ADSL2+.

Try to change transmit power to 50% on wireless advanced page. If it still doesn't help, you'd better test with a different wireless client.

Plugged in DSL connection, how about status of ADSL light on TD-W9980? Since you failed to set it up, did you check Diagnostic result? Provide the screenshot pls.

Upgrade the latest firwmare for your WR740N V4. The current version is very old. http://www.tp-link.com/en/download/TL-WR740N_V4.html#Firmware The Win8.1 laptop is the only wireless client?

No, it doesn't support PoE.

D7 doesn't work with VDSL. You need a VDSL modem router. http://www.tp-link.com/en/search/?q=VDSL