@Kevin_Z 

I already packed it and want to send it back.

However - concerning your questions :

There are several devices connected to router - my PC, NAS, extender to other devices and all of them are affected.

The connection drop is happening during the time when I'm working on PC, so I doubt that it may have anything in common with power management, as I see no reason why my PC should turn off this device while it is working (not in sleep mode etc.) ??
And in the meantime other devices are connected as well.
With my previous router I had no problem with the same configuration.

I can't test it during the day as we need to work and during the evening it may happen that it will work for 2 hours without problem...and then the very next day during working hour it will fail - and it's a problem, because it takes several minutes to restart /reconnect and all of us are disconnected and waiting...

Rgs

Jan

@thanosP I have recently purchased the VR400 V3 modem router and although I have not seen the factory reset issue the LAN ports have stopped working twice in a week on the second occasion I had to restart/power down the router twice to get them to come back up. I was first alerted to this when I couldn't connect to my TP-Link EAP110 which is plugged into 1 of the LAN ports via a Cat5e cable on the other side/ground floor of my home. At first I thought the EAP110 had just crashed but after restarting it there was still no connection. I then powered up my PC which is connected to the VR400 V3 via a Cat5e cable and this also had no network connection neither did another pc in another room also connected via cable to the VR400. Then I went to the router to find the LAN light was off even though there were 3 devices now powered on 2 x PC's and a EAP110 wired access point. Power cycling the VR400 fixes the LAN. I have also found another couple of issues with the device, one being connectivity problems between a 2.4ghz wireless device i.e mobile phone connected to the wireless radio in the router talking to another 2.4ghz device that is connected not to the router but to my EAP110 on the other side of the house. If I connect to the 5ghz network in the router then I can connect perfectly to any device connected to the EAP110. And the other issue is the device will not retain manually entered IPv6 DNS servers you can enter them and save them but after a while sometimes instantly they are removed and left with :: in the box, manually entered IPv4 DNS settings are retained as you would expect.

@TP-Link Sometimes you can ping the devices other times the request times out but the response is really slow always above 200ms and on a persistent ping there are a lot of timeouts. I noticed that if a device is connected to the EAP110 the internet speed is approx 50% of a device connected directly to the host WiFi on the VR400 this only seems to effect devices connected to the EAP110 WiFi on other cabled LAN devices such as a PC the internet is full speed. 2 devices connected to the EAP110 really struggle to talk to each other as does my wired PC trying to communicate with a device connected to the EAP110 WiFi.

DNS servers entered on the VR400 under Network>Internet>Advanced Use the following IPv6 DNS address box is ticked and addresses are entered next to it.

Last night I disconnected the VR400 and went back to my old Router to test and eliminate any issues with the EAP110 and everything functions perfectly.