The IPv6 tunnel can't be configured until one IPv6 connection is established, so if you reboot the router, the IPv6 will back to be disabled. This is the cause of your issue, it is normal. It is stated on the top of TP-LINK IPv6 Tunnel page.

Hinata wrote

The IPv6 tunnel can't be configured until one IPv6 connection is established, so if you reboot the router, the IPv6 will back to be disabled. This is the cause of your issue, it is normal. It is stated on the top of TP-LINK IPv6 Tunnel page.

Thanks for reply....Yes I know that and its also written on the WD8980 IPv6 tunnel setting page that on every reboot this page has to be re-configured, but my question is why I have to do that every time , there should be one autodetection option for IPv6 tunneling with that option modem should automatically detect and enable proper IPv6 tunneling setting on every reboot.

On my two years old netgear DGND3700 v2 , I have this option and its working ......on netgear all I had to do is just enable IPv6 Autodetection and its set permanently.On every reboot after wan connection established its checks and enabled IPv6 tunneling automatically , no need to logon into webgui again and again.......

WD8980 , a product marketed as IPv6 ready with IPv6 implementation is not so good ....there are many product currently available that already have many types of IPv6 tunneling mechanism implemented in the firmware , where as WD8980 only implemented 3 types (as of current firmware) DS-Lite , 6rd and 6to4....A good hardware although lacking good firmware. Waiting if anyone created some custom firmware .
Thanks.