@AndriusVs 

Id first start with swapping out Ethernet cables, with different ones. Factory reset the router by holding the reset button for 15 seconds. Visit your product support page on our main website to make sure you have the latest firmware build.

Another thing to test/try is the NAT Boost, go to Advanced > System Tools > System Parameters, look at the status of NAT Boost. If it is on, disable it, save, and test the speeds, then vice versa to see if any of that helps.

Yes, NAT Boost did the  job and now download 850 mbps, upload 890 mbps , but still 100 mbps less than direct lan -> pc connection.