Deco X20 Backhaul feature not working
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I have six Deco X20s (all six say "US/1.26" on their S/N sticker to the right - which I think indicates the hardware version). They are connected exactly like the attached drawing but with 3 of the 5 X20s wired on the far right in a star from the Cisco Ethernet switch I'm using. [Meaning, where the attached drawing shows "Other Deco Units" I have three wired using X20s using Ethernet cables and I have two that are wireless to the Main.]
My problem is, I cannot get the Ethernet Backhaul feature to work. It used to work fine. I could previously view the settings in the App and the three wired ones indicated "Ethernet" (not WiFi) to the Main. Now suddenly with no changes made by me, they all say "Wifi" and all, at very low antenna power levels because they are a long distance from the Main Deco. Maybe a new firmware update has removed or changed how the automatic/"automagic" Backhaul feature enables itself? I wish I could manually tell the three wired devices to always use the Ethernet cable, but this does not seem possible? Maybe if I log in through a Terminal I could do this (I'd need help with the commands)?
I again followed the Backhaul instructions carefully from the tp-link website by first establishing a WiFi connection with all 5 X20s physically near the Main Deco. Then, after WiFi is clearly working close by, I then moved the 5 X20s to their final, distant destination. Yet, even the three wired X20s never use the Ethernet Backhaul. I know that the cables are working and good because I've used them previously for at least one year, and I have both (1) Link lights at both ends (when I sub in devices with such lights), and (2) I (again) tested the cables with a (simple) Ethernet cable tester. What else can I do? I have changed nothing on the switch since it previously worked, such as: I know I have auto loop detection turned off on it.
Help me Obi-wan Kenobi, you're my only hope. : ) 
You can't force Deco to use Ethernet for backhaul, if it can't do it on its own. Deco Ethernet backhaul discussed here: General questions about Ethernet Backhaul feature on your Deco
You said all your Satellite Deco wired to the switch, which is more than one, lost Ethernet backhaul. That points to one of two possibilities: Ethernet cable between switch and Main Deco, or switch itself.
This is what I recommend you to do:
1. Unplug Ethernet cable that is between Main Deco and switch, from both ends. Then, plug it back at each end and make sure it is plugged tightly. If you have more than one available Ethernet port on switch, you can try plugging cable to different Ethernet port.
If that solves the problem, it was cable that was either unplugged accidentally or its connection to Deco/switch got loose.
2. Unplug switch from Ethernet cable that goes to Main Deco, bring one of Satellite Deco to where switch is, plug that Ethernet cable to Satellite Deco. You will have the following:
Check what that Satellite Deco reports for its backhaul. Possible outcomes:
A) Satellite Deco reports Ethernet backhaul - the problem is with the switch;
B) Satellite Deco does not report Ethernet backhaul - the problem is with the Ethernet cable between the switch and Main Deco.
Ok, I finally returned to this network and am still scratching my head. But at least I learned more.
My first head scratcher: when I arrived, one of the X20s was using wired Ethernet. Yeah! But, ten minutes later after touching and changing nothing, this same Deco was back to WiFi all on its own. (See attached screen shot of Backhaul working at first).
I tried your two suggestions, thank you, they are excellently clear.
1) I unplugged both ends of the Ethernet cable between the Main X20 and the Cisco switch, and after ensuring good re-termination (Link lights on Switch at least), there are still no X20s using the Backhaul (Ethernet) connection. All remained on WiFi. And, yes I tried a system reboot here and a Network Optimization first.
[I should have tried a Traceroute thru the system to see if I could get thru the switch. I'd guess I could since, at first, Backhaul was working!? Next time I'm there I'll try it.]
2) I moved one Satellite X20 to where the Main X20 and switch are. I used the same cable, unplugging it from the switch and connecting the cable directly between to the main X20, and Ethernet Backhaul worked. Yeah! I tried a different cable between the two and it again worked. I then connected it thru the switch. And, no go. So, clearly my Cisco switch is the problem.
So, I double checked that I have Loop Detection turned off on the Cisco switch (see attached screen shot), yet Backhaul does not work, or at least does not work well or consistently with my Cisco switch. Maybe Cisco's Spanning Tree (even though it's disabled) or some other Cisco protocol or other minutia is creating problems for Deco Backhaul.
My hope is that I can buy some TP-Link, 8-port switch, and it will restore Backhaul to the functionality that I need. Namely, I NEED Backhaul to ALWAYS work. My Decos are simply too far apart for WiFi to work well. Next time I will certainly NOT choose X20 devices again and I will require devices that let me FORCE wired connection to function. Maybe TP-Link offers such a WiFi 6 product.
Thanks for the help!
