@David-TP Thank you.  Yes, that connection between the 2 dumb switches is just CAT6 ethernet cable (100 feet or so).

Have you ever heard of cases like mine?  Where it works fine but then occasionally just goes belly up and the Deco's have orange light?  

Also, no problem doing backhaul over "non-dedicated" ethernet?

  @relayman 

I would like to interject into this conversation with a suggestion. Would you please swap Main Deco and Satellite Deco, so that Main Deco is on the left side of the picture. For Deco mesh in AP mode that would not make a difference, but if what I and @David-TP suspect is correct, after that change you would only see one Deco going down occasionally, the one on the right side of the picture (now Satellite).

Also, with that change, there is possibility Deco mesh will recover on its own after outages.

Of course, it is also possible that after that swap you'll have stable Deco mesh.

 As for your other questions, yours is appropriate use of backhaul. Different TP-link product will not make the difference. 

 Hi @Alexandre.  I had that arrangement initially.  Same problem occurs. 

Has behavoir like this been reported before?  

  @relayman 

I haven't seen something exactly like what you describe.

Main Deco requires good and stable Ethernet connection. If Main Deco loses connection to ISP modem/router, different issues may occur. For that reason I would recommend to keep Main Deco as close, network wise, to ISP modem/router (barracuda F93 firewall in your case) as possible.

100ft is quite a long Ethernet cable, still within the specs, as long it is a good one. I really expected that when you move your Main Deco to the left side of the picture, only Satellite Deco would keep experiencing the issue. That would be logical if cable or switches are suspects. You are saying both Deco still go down.

If having Main Deco on the left side of the picture changes nothing, meaning both Deco still go offline, there must be something else I am not familiar with. 

To eliminate switch as a suspect, connect Main Deco directly to F93 and see if problem goes away. In that setup, you may be able to use Barracuda management pages to see how Main Deco appears when it is healthy and how it does when it is not, especially what reported about its link to F93.

To eliminate Barracuda F93 as a suspect, such as not handling well Deco Ethernet backhaul communication, try connecting Main Deco to it in the following order and see if that makes a difference:

F93<--Ethernet-->Main Deco<--Ethernet-->Linksys switch (from the left side of the picture)