It looks as though the beta firmware has been replaced by a stable firmware, dated 07/05/15, I've not downloaded/tried it yet.
description:
New Features/Enhancement:1.The prefix length of the Static IP of IPV6 now can be changed from 0 to 128.
2.Updated the ISP information list.
3.Improved the performance of Wireless 5GHz in short distances.
Bug Fixed:1.Fixed the bug that the speed dropped on the BT line of UK when G.INP was enabled.
2.Fixed the kcodes security bug.
3.Fixed the bug that TD-W9980 had compatibility issues with the line of ZEN in UK.
4.Fixed the bug that TD-W9980 had compatibility issues with the line of TALK TALK in UK.
5.Fixed the bug that static IPV6 connection did not take effect after a reboot.
url: http://www.tp-link.com/en/download/TD-W9980_V1.html#Firmware