Hello Vkuznetsov, on this case a recommendation will be to update the firmware of the router and also confirm that you have the following settings at your wifi adapter:
What are the recommended settings for 802.11n connectivity?
Also I suggest you to take a look at the following links:
What are the different Wi-Fi protocols (802.11a,b,g,n) and data rates?
Data rate will not exceed 54 Mbps when WEP or TKIP encryption is configured