@KimcheeGUN 

Thanks a lot for your recommendation. In fact I enabled only 5GHz radio mode only as I assume my clients all support it. More over before once I enabled 2.4 mode my TV screens failed to connect as were not able to decide to which mode they should connect. So I created hidden 2.4 SSID just for TV screens.

Regards signal strength here it is the top 10 clients (signal strength arranged from lowest to highest). You said I can try to enable Fast roaming option. Ok I will try it. Perhaps I should also try enable mini rssi at =...., so they can get booted to the closest AP. Or perhaps I should check what APs these clients connect to and raise up its signal power level? 

  @Vincent-TP 

Hi and thanks a lot. Today I've found that when I walk inside my space and obviously should be switched between APs I have an issue: I launch ping command with -t key and when I walk I get lost reply one of approximately 10 pings. I suppose it happens when I switch between APs and it doesn't happen smoothly. So if I get ping packets lost then any voip call will get a short break. My APs are overlapping but it seems the roaming is not configured properly. I thought Fast roaming option can help here but heard that in many cases such option enablement brings new issues. ANd I made ping test with minimum RSSI signal enabled and disabled with the same result.