WiFi speed down from 300Mbps to 54Mbps??

James7679 said:

on the other computer that is connecting wireless, are you having the same issue? If you are, the router could be failing.

Do you have the same connectivity issues while connected to other wireless connections?

Are you sure that no one else is connected to your router? Maybe change the password to something very complex and see if the issue persists.

Sorry, I have to clarify there are 3 clients connected now:
1 Desktop: CAT5e, <-- Normal
1 Laptop: Wi-Fi-G (54Mbps), <-- Normal
1 Laptop: Wi-Fi-N (54Mbps) <-- Problem!

I have these 3 PCs only but no any other 300Mbps devices, Thanks.

This problem is present even the laptop is next to the router. Both 2.4 & 5 are set to "Auto" (Only 2 choices: "20 MHz" & "Auto")

Run a program called InSSider and you'll be able to see what channels everyone round you is using and how strong they are. 300Mbps is much more likely than 150Mbps to be affected by overlapping usage of channels by nearby wifi users, so it may be worth changing to 150Mbps and setting your channels to avoid other users as shown by InSSider.

Thanks for everyone!!! Problem solved now by changing the setting of "Mixed-Mode Protection"