Thanks for the ideas!
I have changed MTU in ND as showed on the below picture but didn’t help, even after a reboot on the client PC side then on the router side as well.
Yes, native IPv6 would be better but as a Hungarian Telecom subscriber I can get native IPv6 address only via zero configuration. My Windows PC can get IPv6 address directly from the modem but I could not configure my Mikrotik router to do so.
