Hello @MarufAhammed 

Thank you for reaching out to us and welcome to the TP-Link community.

Can you please describe the speed drop issue in detail? What is the correct speed you can get when connecting to the C64, and what is it when it drops?

Does this happen to the wireless connection or do both wired and wireless have the same issue?

If the speed is not normal after updating the router firmware, it's suggested to factory reset the router and try reconfiguring it from scratch, then check and monitor the speed again.

You can also go through the troubleshooting suggestions provided in this article to exclude some interference.

@MarufAhammed 

Thank you very much for getting back with the detailed info.

Just want to confirm again does the speed issue occur just after you upgraded the router firmware to the latest? Before the update, is it working fine?

Is it convenient to confirm if you can still get a good speed if you connect to the ISP modem directly and dial up on the computer? If yes, please remove the C64 from the network, and connect a computer to the ISP modem directly, then follow the guide below to dial up on the computer to monitor if you can get the desired speed:

How to set up a PPPoE connection on Windows 8&10

How to configure PPPoE connection on Mac pro

If the speed is always good and stable, then connect the modem back to the WAN port of the C64, and reconfigure it to connect to the internet to monitor the speed.

@MarufAhammed 

Thanks very much for sharing all the detailed info.

To address the issue and try to fix it, we would like to have a specialist look into this further via email. Please check your mailbox later, and let us know if the issue is resolved.