@kklim 

Hello,

Please verify some basic information and we can try to fix it:

1. Is the main Deco working as a Router or an Access Point? Is the second Deco connected to the main one via wireless?

2. Try to connect your computer to the 2nd Deco M4 by an ethernet cable, then run a speedtest to test the wired connection.

3. Relocate the 2nd Deco and move it closer to the main Deco, then test the speed via speedtest again.

Good day.

@kklim 

Hello,

How about the wireless link speed on that computer or phone when connected to the 2nd Deco M4 on the ground floor? Please confirm it as below:

https://www.tp-link.com/support/faq/2265/

If the wireless link speed is slow, try with another wireless device and confirm again, tell me the model number of the wireless adapters.

Additionally, try to move the 2nd Deco M4 closer to the main Deco and ensure there are no/fewer obstacles between them. If it doesn’t make sense, reboot all the Deco units for a try.

@kklim 

Hello, thanks for all the details.

This case will be forwarded to the engineers and they will evaluate if it needs to be replaced.

Please check your email box and they will contact you via email, thanks.