@Fabson 

Do you run your Deco mesh in its default Router mode, or have you changed it to Access Point mode? The answer will help with troubleshooting.

If you are unsure, check this FAQ:  How to set up Deco to work in Access Point mode

Don't make any changes in Deco app, this FAQ document should help you locate configuration setting responsible for Deco mesh mode, to give me the answer.

  @Fabson 

With Deco mesh in Router mode you can't swap Deco units (Main and Satellite) also can't connect ISP modem directly to the switch. This will not work regardless of any issue you have on your network otherwise.

With your current network layout, what you have is very similar to what is discussed here: https://community.tp-link.com/en/home/forum/topic/655034

The only difference is the trigger.

In summary, consider replacing Ethernet cable between switch TL-SG108 and Main Deco.

If you tried that already, and it did not fix the issue, please tell me what devices other than Satellite Decos are wired to Ethernet switch. This may help with further troubleshooting steps.

I've found another fix. After reboot (when the wired connection between switch and main deco is not working) I unplugged the Ethernet cable between the ISP's modem and the main deco. After a minute it started working properly.

I’ll try swapping the cable. There is nothing else connected to the switch. All devices that are connected via Ethernet are on the diagram. I suspect that there is something wrong with the IPS modem (as described in the #5 reply). My ISP modem is an old ZTE F660 router with an optic fiber port. Are there any settings in the ISP modem that might cause issues?

  @Fabson 

I can't think of any ZTE F660 router settings which could cause this. 

What you have appears to be an issue between Main Deco and switch. Your switch tells you there is no link between switch and Main Deco. Think of Main Deco as just another WiFi router, which it is. Your nework diagram is the following:

ZTE F660 router <-Ethernet->X10 router <-Ethernet-> TL-SG108 switch

You must agree that whatever happens on ZTE F660 router is unlikely to impact wired link between X10 router and TL-SG108, to the point where switch LED reports no physical link to X10 router.

Out of all possibilities, these are most realistic:

1. Issue with Ethernet cable between Main Deco X10 and TL-SG108.

Except, still not clear why it is not persistent and requires equipment restart to trigger it.

2. Issue with TL-SG108 switch not handling well Deco Ethernet backhaul

But in that case, both Satellite Deco won't be able to communicate with each other over switch, while one of them connects wirelessly to Main Deco.

3. IP conflict between Main Deco and device (other than Satellite Deco) with static IP address connected to switch

You said nothing else is connected to switch

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This is a bit puzzling, but if you replaced Ethernet cable and it did not fix the problem, also try connecting it to different port of TL-SG108 (to address very remote chance single port failed on switch).

If that does not help, and you are willing to redo your network layout to minimize issue and/or narrow down where could it be, I can give you steps.

  @Fabson 

One more thing. If your Deco app allows to configure assignment of WAN Port, please do that for X10 Ethernet port which is connected to ISP modem: https://www.tp-link.com/en/support/faq/4252/

This is the only thing I can think of on ISP router which could impact Main Deco connected to it. 

If this feature is not available in your Deco app, ignore that.