@Pepislav I suspect there is a generic IPv6 issue with all current firmware by TP-Link. I just bought a Archer NX200 which works great but IPv6 stops working after a few minutes regardless of the settings I use. My ISP fully supports IPv6 and I had no such issue with another router by a different brand. There are several similar complaints in the forum concerning various routers.

For what it's worth, in my case, if I reestablish the wifi connection with the client IPv6 works immediately again for a few minutes. It's like the router forgets to talk to the client or  drops the route. Maybe the router confuses minutes and seconds with respect to the lease or whatever.

  @ArcherCharlie  It also works for me after turning the antenna on the roof off and on to receive the internet but I can't start downloading, my internet goes out within a few minutes and the Deco lights up red and only turning the antenna on the roof off and on helps. When I have only IPv4 turned on, everything works.

  @David-TP 

Provider : Amigonet

Antenna on the roof: MikroTik RBnRAYG-60 (nRAY60)

Internet connection type: Dynamic IP

I have prefix delegation turned on.

I'm downloading a game on Xbox and in the next room they're watching TV via IPTV and after a while the internet goes out and the red light on the Deco X50 lights up and I have to turn the antenna on the roof off and on to get the internet to work.