I've replaced previous TP Link WiFi 5 mesh system into this one and I'm struggling most of the time.

the biggest issue I cannot solve is that Deco which is in my bedroom is keep on connecting to the Main deco on another floor, where literally 5m, straight line, no obstacles is another deco that is connected via Ethernet with the main one. 
 

it results in speeds I saw as a kid. 2-3mb/s instead of 960-990mb/s. I'm trying to manually force deco to connect to the one I want, but it does nothing. The only option for me is to restart the main deco. When it's offline these two connect to each other and internet is working properly for an hour or two. Then the bedroom deco connects back to the one downstairs. 
 

is it me facing this issue only? Is it fixable? Is it something tp link can fix or I'm better off selling these decos and forget I ever had them?