@BaCsaba 

Your Deco X50s, are they connected to each other the following way:

  @BaCsaba Hi Alexandre,

No, they are all connected via TP-Link TL-SG1024D switch. All the four Mesh devices are connected to the first four ports. Since they are in AP mode, I suppose this setup is fine.

"When the Deco is set to work under Access Point mode, you can wire satellite Deco to any Deco unit or the main modem/router."

  @BaCsaba 

I am familar with the symptoms you describe and they can be triggered under specific circumstances, but for that Main Deco must be connected to ISP modem/router, not through the switch.

Perhaps, there is different network layout which still makes that possible. Instead of me guessing what you have, please share network diagram. 

  @Alexandre. 

I check the connections on Thursday (I am on vacation) but as far as I remember all the four Decos are connected to the switch as well as the ISP Modem. If I am right, I will re-design the network like this:

ISP MODEM -> MAIN DECO -> Switch -> Deco 2

                                                                -> Deco 3

                                                                -> Deco 4

I am suprised that it matters in AP mode but if it's mandatory to setup like this I can accept it.

Regards,

Csaba

  @BaCsaba 

Please double check.

Here is the thing. If you have network diagram similar to this, I  have good explanation of where the issue is and how to troubleshoot it:

The problem will be between switch and Main Deco: the Ethernet link between them is broken. All symptoms match.

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If, on the other hand, you have the following network diagram, I have no idea what would trigger the issue you described:

  @Alexandre. 

What do you think, what is broken between them? Since two of the three Decos are connected via Ethernet it seems that LAN connection is ok. Please let me know where is the issue and how to troubleshoot it.

  @BaCsaba 

Take a look at this network diagram and assume there is more than one Satellite Deco hardwired to the switch:

What will happen if Ethernet link between the switch and Main Deco is broken? For example, if Ethernet cable between them gets accidentally disconnected.

In that case, one of Satellite Deco will establish WiFi link to Main Deco. All Internet traffic that goes through the switch will pass through that Satellite Deco to Main Deco. You will still have Internet access, perhaps slow but not down. It is network resiliency that Deco mesh provides.

What will happen if you reboot Satellite Deco wirelessly connected to Main Deco? Satellite Deco will briefly go offline, Deco mesh will notice that and another Satellite Deco will take its place. You will see that another Satellite Deco now reporting WiFi link to Main Deco.

What will happen when Satellite Deco you rebooted comes back online? Because there is another Satellite Deco already providing WiFi link to Main Deco, the one you rebooted will connect to the switch by Ethernet.

The fix is quite simple: find what is the problem with the Ethernet cable between switch and Main Deco, and correct it.

----------------

As you can see, all symptoms match perfectly, but you are telling me this is not the network diagram of your network. This is quite strange.

I can only think of another network setup where issue will appear and all symptoms will match: Main Deco connected directly to ISP Router Ethernet port, all Satellite Deco connected through the switch, the switch connects to the different Ethernet port of ISP Router, and that Ethernet connection between switch and ISP Router is down.

That is why I asked you to double check what do you really have with your network.

  @Alexandre. 

Yes, what you describe has logic but as the original post said the problem is that I have two Decos which looses Ethernet backhaul connection. When "Lilla fszt" looses eth backhaul and I do a reset on that particular mesh then it connects back on ethernet however suddenly "Balint" deco looses the ethernet and connects on wifi. Doesn't make sense that it's a cable problem in this case.

From other perspective it has no effect if the Main Deco is on the "left side" or on the "right side" of the picture, it's connected to the switch as well as other Decos :)