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...

@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...