May I please ask for help with a very unusual behavior of WiFi of Deco S4 using 6 devices?

Problem is with WhatsApp and Deco S4 setup with 2.4GHz access. Problem is intermittent meaning that it occurs maybe 70% of the time with no obvious reasons why.

Internet source is not the reason which I checked. S4 network is accessed through WiFi with Android phones made by Samsung. We are communicating using WhatsApp.

The voice only calls are frequently disconnected, while video calls have video feed stopping with voice uninterrupted. That in itself is strange.

When the video stops - the super sharp image remains on the screen, until anther image reappears fraction of a second later.

There is no smooth video when that happens. It is weird, because with insufficient bandwidth the WhatsApp usually reduces video quality and image pixelates.

This is not happening. It looks like WhatsApp is transmitting picture after picture during call, not a video feed. 

I check the internet connection and it is good and stable, I checked the phone with cellular data and works fine - it's just this Deco S4 WiFi.

Also - I have an ancient router on the internet line and if I connect directly to that WiFi (it is a different network) then all the calls are fine.

Thanks in advance for any help.