There is misunderstanding... HiSpeed is asking for model number of the main router instead of the access point. There is a router connected to LAN port of WA901ND, isn't it?

Are you trying to boost Wi-Fi signal in the same floor or different floors as W8970? In different floors, powerline extender kit should be better.

If you turn off the range extender, can you connect iPad and iPhone to the main router's Wi-Fi successfully with the same password? Try to forget the network and reconnect. What's the firmware...

What's the ISP connection type? Have your tested the internet connection by a different modem or contacted your ISP on this issue? Try to disable UPnP.

Your firmware is not the latest version. http://www.tp-link.com/en/download/TD-W8968_V3.html#Firmware

Is it possible for you to try on different computers? Disable anti-virus and windows firewall.