Thanks @robrbaron for your reply but I don't use 40 Mhz, I tried with Auto and 20 Mhz but no change.

I am getting very low speed on 2.4 GHZ. I have channel setting Auto and width 40 MHZ. I have 100 mbps connection and getting good speed on 5 GHZ but on 2.4 GHZ I am getting only 20 mbps speed....