@Milo89 

If your routers are AX55 V1 models, then you do not have the latest firmware.

  @jra11500 Thank you. I have checked this just through the webapp. I see there are later versions manually installable. Will try.

Topic updated. I have picked wrong model - it is ax53, so it means the firmware is the latest.

  @Marvin_S 

1.  ISP router -> switch -> First floor AX53 1 (AP mode, ssid1) 

                                         |-> second floor AX53 2 (AP mode ssid2)

Third router is is different building connected to different isp router without switch in AP mode

2. All wireless, 4 connected continuously, max 10 wireless when I have guests. When 5ghz is off usually the newcomers are unable to connect. Does not matter if it is ios or android. I have tried to disconnect myself during visit and I was not able to connect also. 

3. Tried smart connect, same or different wifi names. Behavior was the same. In general : 5ghz smart connect enabled - micro drops in connectivity. 5ghz disabled : unable to connect for some devices

4. 5ghz, smart : twice per day 2.4 g only - almost each time there are visitors (does not matter if new or already known to wifi router)  have not tried - I have wireless only

5. Troubleshooting: tried these tips also tried to change encryption, channels etc. No results for both buildings