@arbytronic I had to give up and sent the router back and got a refund.

I have the problem with 5 GHz wifi. After a few days i get packet loss and ping times of more than 2000ms.

With 2.4 GHz the problem doesn't occur. So I have now disabled 5 GHz. But it would be good if there was a Firmware which solves this problem, because 5GHz WiFi is not usable for me.

I gave up and returned my RE305s. I Still use the MR600 and wifi works a little bit better but still drops connection and interrupts wifi calls even when reception is good and I'm close to it. So I will disable the wifi on it and replace it with something else and only use the MR600 as a 4g modem.

Thank you for all your feedback. 
A small update from my side:
Some time ago I loaded new frimware on RE305 and this problem almost disappear. Although, I think that the signal is worse and still experiencing problems with quality of the connection. Yesterday, I spotted new firmware to RE305, so I loaded and after that my RE305 crashed, ie only power diode is on and no connection with extender, even with cable.

Today I sent it already to the service. 
In the meantime I bought RE650, which has more antennas and should have stronger signal. There is also a new firmware (finally ?) for Archer MR 600, which I'm going to load soon. 

I will let you know how it works....

Looks like TP Link does not rush with delivering fix... perhaps, they have problem reproducing this bug? Nevertheless, would be nice if someone from TP Link could reply here.

What if I bought this router almost 1 year ago? TP-Link will refund it? This problems has started around 06.2020. All time users in Wifi network are beeing disconnected.

@Mozart Last time I has made update firmware to latest build 200511 and the problem still occur. Quantity of wifi users 11 and 5 wired.