@Ankush_Goswami 

Hi, we will do some troubleshooting to locate this issue and try to figure it out. 

1. What is the current network topology? 

2. Standing next to the Deco on the first floor, can you get a good and stable wifi signal from the Deco on the ground floor. (vice versa, which depends on where the main Deco is located)

3. Have you ever tried to swap the location of the Decos or move them closer to each other? 

4. You can also change the slave Deco to the main Deco and give it a shot. https://www.tp-link.com/support/faq/2400/

Good day.