How can I set a static IPv6 address for a WiFi connection?
Open a terminal in your android or connect to your smartphone from pc with "adb shell" command. Then type "ip a" command, just to know how your needed interface is called. For me it is "wlan0 ".
Now, type the following command  

 # ip -6 addr add 2001::your:want:ed_s:tati:c_ad:dres/CIDR dev interface  

For example,    

# ip -6 addr add 2222::1234/64 dev wlan0  

Now execute "ip a" again or go to settings advanced and ensure, that your wanted address was assigned.
As I can see, this method do not survive rebooting. For permanent configuration you need use another way.