@HelpFixDecoApp thanks a lot for the advice. I'll try that too, although I reckon the issue is most likely caused by some problem between the ISP router and my main Deco BE85 directly connected to it.

Yesterday I spent the whole day as well as part of the night trying everything I could think of to try and fix the issue, including temporarily shutting off the two satellites Decos (the BE65 and the second BE85).

I also tried switching my Decos from AP mode to router mode, although the router mode would not be the ideal setup for my local network, but no matter what I do, every 10-20 minutes the Internet connection is lost on my Decos and then resumes after another 10-20 minutes.

I also checked the troubleshooting fixes suggested by TP-link, but none seems to apply to my situation. Indeed there are no issues with my router ISP connection and the cable connecting it to my main Deco is brand new and worked well until very recently. However, today I'll try with a new cable just to rule out that too. Other fixes suggested by TP-link, like using Google's DNS, seem only to apply to the Decos in router mode, which is not my case. Unfortunately there is not much I can do even on my ISP router, because its settings are quite limited.

While writing this message I also noticed my wireless speed dropped from about 1 Gbps to about 2 Mbps both in download and upload. After a couple of minutes I lost the connection again then it was back with a speed of about 150 Mbps, still much slower than usual.

  @HelpFixDecoApp 

I was hopeful that was the case but apparently it wasn't. Same outcome even using a brand new cable to connect the main Deco to the ISP router unfortunately.

It seems like there's a clock somewhere that shuts downs the Internet connection for the Deco network at regular intervals (20 minutes more or less). If I restart the ISP router the connection on the Deco devices is resumed almost immediately but after about 20 minutes is gone again. 

I'm startled by the fact that there was no change on my local network, none that I'm aware of at least, that could explain the sudden change of Decos behaviour.

  @HelpFixDecoApp 

Still looking into the Deco BE65 firmware upgrade. It took me a while to understand how to manually install it, as I thought I could manage the Decos only via app but now I see there is also a web management interface available.

I'm still a bit hesitant because when I downloaded the beta firmware zip file, Tp-Link website warned me about using only firmware files available on the website of my geographical area (Italy/Europe), otherwise there could be some warranty issues. And the beta firmware for the BE65 is not available in my area.

  @HelpFixDecoApp 

I appreciate that, thanks. At this point 10 more hours are not a great deal to deal with. Better safe than sorry.

I'm now clocking the exact uptimes and downtimes of my Deco connection to check if indeed they are always lasting the same time, but that's the impression so far.

I've also started thinking about performing a factory reset of my Deco network, although I will wait until tomorrow at least before doing that.

It won't help fixing the problem, probably, but out of curiosity I monitored more closely my Decos Internet connection over two hours. 

During this timeframe, the connection dropped four times, for an average of about nine minutes, while the uptimes lasted on average about 24 minutes.

To monitor the connection status I played continuously some YouTube videos, which carry on playing for a few minutes after the connection drops, therefore the real uptimes were a bit shorter.

Unfortunately I have still no clue about why this is happening. When the connection is working everything seems normal, although I've randomly noticed some drops in the wireless speed.