@Srikrishna.M Exactly what I have suspected: That's the reason you don't see speeds over 10 mbps when you connected to C6 router. Change the ethernet cable between your modem and the C6 router with...

@iamalone If your router is EU version then this is the last FW release.

OK, now lower the transmission power of both wireless bands to medium or low and test again.

If you don't have any special configuration, would you try to hard reset (Method 1) the router and re-configure from scratch, then test if that will improve stability.

@Srikrishna.M What device is sitting before the Archer C6 - ISP cable modem, ISP router, etc. ? Capture & post the following screenshot:

@HP15z Follow this reference to use these features.