For about two years now I've been using the TP-Link Deco X20 (3 nodes) without a problem. My setup used to be as follows, where the Deco system is running in Wi-Fi router mode.

Since I needed more wired network connections near my modem (where the first Deco is located), I've decided to add a switch in between. The setup is now changed as follows. The setup isn't exactly as described in the documentation provided by TP-Link, as I've attached the secondary X20's in series rather than in parallel. This was necessary because of the location and possibility of running ethernet cables.

I would assume this setup would work as it used to, but it doesn't. I've read about some switches not allowing ethernet backhaul, and I've also read about loopback detection blocking the feature. I've looked up the loopback detection settings in my switch, and found it is disabled. All settings are factory default for now, and I've found nothing in the manual to suggest it would block ethernet backhaul functionality. My switch is a Cisco SG220-26P (managed). For those who are interested, I've attached a PDF manual for the switch.

I've not tried replacing the managed switch with an unmanaged one, as the one I've currently got is in use. I will try this soon, if I find no other solutions. Does anyone know of any settings I can look into that would block the functionality, or any tricks I can try to see if it will work with my current setup?