@Mapper 

Hello, we need get some information first so that we can do further analysis and figure it out. 

1. The current network diagram. Especially how do you connect the slave Deco to the main Deco. Is there a cable from main Deco to the slave Deco directly, or there is a wall socket in between?

2. When Deco stops working, have you ever tried to connect your computer to the main Deco/cable directly and test whether it works? 

3. You can swap the locations of the Deco units or change the slave Deco to the main Deco.

https://www.tp-link.com/support/faq/2400/ 

May it help. 

@Kevin_Z 

1. Nothing in between, direct cable, no wall socket

2. No, I don't, there is no possibility to do so.

3. I will try when I get home.

More observation:

When I disconnect bachaul cable on Thursday evening network was more or less stable during weekend. No wan dropping, no red lights blinking. Hovewer latancy on devices connected to slave is awfull most of time. Probably this is because of slave position and not working cable backhaul.

@Mapper 

Thanks for your reply. 

Generally speaking, the wired connection is more stable than wifi connection, it is normal that the latency of wifi connection will be higher than wired connection. 

You can change another ethernet cable to connect the slave Deco and main Deco, like cat 5e or above. And please confirm whether you can access internet from the WAN cable (internet source) directly when the Deco stops working.

Besides, take your time to test the performance after you swap the location or change the slave Deco to main Deco. 

Any updates, be free to let us know. 

@Kevin_Z 

Okay, so I switched places master and slave, and did connect them with a cable again. Generally speaking situation is same. After they are booted up, slave shows that it's connected over wifi while cable is in place, after some minutes app isn't able to get any info and kicking me off to "we couldn't find deco". So it behaves exactly same way it was before. Neither I am able to get to web site to download log.

I've disconnected cable and there are bunch of "unable to connect to 192.168.1.100"  (see attached log)

So I got them together and link with a patch cord. Situation is same Ethernet backhaul isn't working and after couple of minutes application kicked me off.