Hi @shawonarif 

Could you please specify if you encounter some issues when trying to downgrade via Archer C64's web-interface?

Please provide screenshots if there are any errors appearing.

Thanks!

@shawonarif 

I'm agreeing with @woozle at this point, because I haven't seen any changes on Wi-Fi coverage neither. 

At this point you should try resetting your router to its default settings. 

If the issue persists, then you can also try changing Wi-Fi broadcasting channel so it less likely would interfere with other Wi-Fi networks. To do so, firstly, scan Wi-Fi networks around (there are a lot of apps for this in the internet) and find the least used channels for both 2.4 & 5 GHz. After that, log into your router's web-interface and go to "Advanced" > "Wireless" > "Wireless Settings" where you can set the preferred channel. 

P.S. for 2.4 GHz I would personally recommend using 1, 6 or 11 channels because they have less interference with other channels.