So, finally my connection was fixed and fiber plugged into XGS-PON splitter.
I have 8Gbps contract.
But, no matter what I do (iperf3 or speediest, which iperf3 server etc), I can not get speed higher than 3Gbps. Mostly it is around 2gbps.
My router, for sure, can handle more than that.
What can be problem here?