@IgorBljahhin 

Have you started to have frequent power outages in your home recently? If yes, check this forum topic for similar discussion, and my posts there for root cause and workarounds: https://community.tp-link.com/en/home/forum/topic/572762

  @Alexandre. absolutely no any power outages in years

  @IgorBljahhin 

A 192.168.68.x  address usually indicates that the Deco is in ROUTER mode, this is the default address for the Deco out of the box.

If its getting anything outside the 1.yyy range that your router is providing then my guess is one of the Decos is still in router mode..  how that happened or what in not sure but seems that way

When you get the 192.168.68.yyy address did you check what the default gateway is?  likely 192.168.68.1    my guess is that would be your culprit for the issue

Could be one of the Deco device is going back to router mode or something like that.. cant think why how how, but what you describe sounds like it

IgorBljahhin wrote

  @Alexandre. absolutely no any power outages in years

  @IgorBljahhin 

I asked you about power outages, because I can only reproduce what you described, with Deco running in AP mode, when both ISP Router and Main Deco were bounced. If someone haven't intentionally or unintentionally restarted them triggering the issue, it must be power outage.

I can explain what is happening, and you can test it on your home network to verify my explanation.

Suppose, your Main Deco runs with IP address 192.168.1.101. When it gets disconnected from ISP router, Main Deco will start its own DHCP server. The feature is called Smart DHCP. Power off ISP Router, or unplug Ethernet cable connecting Main Deco to ISP Router, to trigger that.

In this situation, Main Deco keeps its IP address 192.168.1.101. It becomes default gateway, DHCP server and DNS server. Force device connected to Deco mesh obtain/renew its IP address, and you should see 192.168.1.101 for gateway/DNS/DHCP. Also, device will get IP address from 192.168.1.* range. More to that, if that device was already known to Deco mesh before, most likely it is going to get same IP address it had before, from 192.168.1.* IP range.

While ISP Router is down or unreachable by Main Deco, bounce Main Deco. Power it off and on. This will force Deco mesh forget about your home network and start with default IP range for Deco running in Router mode, which is 192.168.68.*. Only now your devices should start getting IP addresses from that IP range.

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I recommend you check again how your ISP Router and Main Deco are powered. Are they connected to adjacent power outlets, do they sit on the same power bar? 

Often, ISP Router has uptime reported in its Admin web page. If yours does, check its uptime to figure when last time it lost power or was bounced. 

Also, check Ethernet cable between ISP Router and Main Deco, is it tightly plugged? Unplugging cable will trigger the issue, but on top of that Main Deco must be bounced afterwards to force it into 192.168.68.* IP range, so it can't be just cable.

Main Deco starts its logs after each reboot. During the event, try accessing Main Deco at http://192.168.68.1 and check when its logs started. 

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There was someone on this forum who managed to connect Main Deco with ISP Router through powerline adapters. While this is creative, it could be the way to trigger what you described.

  @Alexandre. Thank you a lot for detailed explanation. Now I will wait for next failure of Decos and then I will check ISP router for uptime and Decos for logs.

  @IgorBljahhin 

Also, if your Deco mesh got into this situation, there is easier way to restore it than reset and reconfiguration.  Here is the algorithm:

Power off all Deco nodes, starting with Satellites. Main Deco will be last to powered off.

Verify that ISP Router has Internet access.

Power on Main Deco. Wait till Main Deco M4 LED goes white solid. That should mean it is online and healthy.

Double check in Deco app that Main Deco is online and has IP address from 192.168.1.* subnet

Power on Satellite Deco in any order

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When Deco runs as DHCP server, it assigns IP addresses from 192.168.68.* with very short Lease Time, under few minutes. After you fix Deco mesh, every device connected to it will automatically get proper IP address from 192.168.1.* subnet in less than 5 minutes. 

  @IgorBljahhin 

You may be able to get beta firmware for Deco M4, that can allow to disable Smart DHCP feature. See https://community.tp-link.com/en/home/forum/topic/155575?page=10 , posts #97 and #98.

Note that this is beta firmware, which means it might have other issues. Also, that firmware won't fix the trigger of an event, but if that trigger goes unnoticed perhaps it does not matter.

  @Alexandre. Thank you for the hint. I will ask for firmware.

That's weird, but all troubles with Decos disappeared after I made this post on support forum :)