I find a new firmware for TD-W8960N V7 here. http://www.tp-link.com/en/download/TD-W8960N_V7.html#Firmware Give it a try?

Did you upgrade firmware recently? Here is a firmware I got from TP-Link, you may have a try to upgrade. http://support.tp-link.com/f/23aef3da52/?dl=1

Seems no much difference between the two routers, just the rate. You can change the DSL modulation type of the new router to G.dmt or adsl2+ to have a try. Go to Advanced Setup-DSL, uncheck all...

good sharing!

Did you enable IPv6 on the deco app? You can choose bridge for the IPv6 connection type.

Did you already setup some static IP address on Desktop and Access point? On the desktop , you can have a try release and renew the IP address. For the Access point, restart it as well.