@Amarnath.1980 

There are only two things I can think of.

First, check that you have set the "Channel Width" in the router's 2.4 GHz wireless settings to "40MHz".

Second, verify that your client device actually supports dual-stream (2×2) and 40 MHz channel width operation. To do this you need to check on the client device if it reports a connection speed of 300 Mbps (or at least something between 200-300 Mbps). How you check that depends on the operating system of the client device. For example, on Android you will probably need to install an additional App to get the connection speed.

@woozle 

                Tnx for showing interest on my problem.I have installed "ookla speed test" in my android phone,but it shows only 25 Mbps to 30 Mbps even just infront of my router.I have also set the channel width to 40 Ghz.

@Amarnath.1980 

As it stands, there are still many smartphones around that only have one WiFi antenna and also many of these don't support 40 MHz channel width. If both things come together, then one will end up with just about a quarter of the speed that the Archer C6 is capable of on the 2.4 GHz band.

To find out what's the (theoretical) link speed at which your smartphone's WiFi is connected to the router you could install a small App called "AIDA64". After you've started it click on "Network" and check the "Link Speed" in the Wi-Fi section. That "Link Speed" is a theoretical value. Under real life circumstances one can usually get a data thoughput of about 50-60% of that Link Speed if router and client device are in the same room.

As example I've attached two screenshots from an older phone that I own. (btw, my Internet connection is about 200 Mbps)