You’re welcome @dusanhauko
Warning… got a bit lengthy as thoughts were coming to me so my apologies in advance.
If you make it through to the end, please report back any findings 
Thoughts…
Because of some dropout/connection issues I was experiencing, I had been (painfully) visually/manually monitoring multiple apps at the same time for long periods of time 
:
- Router Device Connection Status
- Shelly Cloud App
- Locally Run Shelly Scanner App (VERY Cool App by/the/way)
What I found was that there was often a “lag” between when the Shelly Cloud App indicated that a device was online/offline (sometimes not displaying a disconnection at all) and an Actual Disconnect (as observed in the Shelly Scanner App and/or Router Device Connection Status). i.e. I could see the disconnect “live” & immediately in the Shelly Scanner App, while the the Cloud App I suspect has a period of time before it reports the offline status.
While you’re in a non-crowded area, I would encourage you to run a Wireless Survey anyway. I have neighboring routers 500+ feet away that would sometimes show up strong.
I’m not familiar with your Router/WiFi/Access Point. Is it an integrated “Mesh” type network that negotiates WiFi Channels on its own, or are there 3 separate Access Points. In either case, they may be competing for 2.4GHz Channels with each other, sometimes causing interference & disconnects. If there is any Auto-Negotiating taking place, those disconnects you observe can be when the units are finding interference and switching channels automatically/quickly. Again, perhaps a Wireless Survey might reveal more info. Consider setting your AP’s 2.4GHz to fixed and separate/distinct channels, rather than allowing them to Auto-Negotiate. I’ve watched nearby AP’s bounce back/forth between channels during their Auto-Negotiate process: It’s like AP A saw AP B channel 3 was interfering so AP A moved to channel 6, while in/around the same time AP B saw channel 3 interfering, so it moved to channel 6 at the same time, resulting in a continual ping-pong.
Shelly INTERNAL WiFi Access Points & BlueTooth: Do you have those enabled? If so, I think their default WiFi Channel is 5, which could be interfering with your AP. Once the Shelly Devices are set up, their internal AP can be disabled. Same for their Internal BlueTooth Connections, they share the 2.4GHz Bandwidth and could/should be disabled. Unless of course you’re using either/both the Internal Shelly AP and/or BlueTooth AP for direct connections for something else.
Shelly WiFi Roaming: My Shelly Gen1 Devices are currently offline, so I can’t check, but do your Gen1 Devices have WiFi Roaming Enabled and what is the Threshold set at (WebGUI/Settings/WiFi/WiFi Roaming)?
Shelly Connection Strength: What are the RSSI’s for each of your Shelly Devices (found in WebGUI/Settings/WiFi
If you install the Shelly Scanner, you can see them all at the same time and watch them LIVE for a few minutes:
