@m9defective 

Hey

This is likely not an issue with the Deco, rather an effect of adding a switch to the configuration.  Let me try to explain.

The Deco range has TWO operating modes, namely  ROUTER mode and ACCESS POINT mode.

Router mode is the default mode of operation, basically one of the Deco nodes acts as the master (one plugged into the ISP Modem Router) and the rest act as mesh devices.  In this mode the ports on the Deco devices are used for CLIENT connections, namely something like a PC and the nodes communicate over WiFi between eachother.   Plugging the nodes together into a switch is not recommended as they will all treat this as a client device, it could cause network traffic floods and that would slow the performance.  In short the ports in Router mode are not for connecting the Deco devices together, they only service client devices.

If you wish to use the ports on the Deco devices to allow the nodes to communicate, you need to switch to Access Point Mode.   This will change the ports from Client Only, to Backhaul and Client.  Therefore you can plug all 3 decos into the switch and they will talk via the switch and not over the WiFi.  Bear in mind that if you are in Access Point mode, all decos need to be cabled in.. you cannot mix cable and WiFi backhaul.   Any Deco without a cable connection will not work in Access Point mode.

Hope that makes sense and more info on the different modes here

https://www.tp-link.com/us/support/faq/1842/

  @Philbert 

Thank you for the explaination. That makes sense, and I did initially mix both wired and wireless backhaul. But I got tired of moving ethernet cables around while troubleshooting so my deco's have been connected wirelessly since. My setup is as follows:

ISP Modem (Bridge mode) with only one ethernet out to the master deco. Master deco connects to ethernet switch (TL-SG108) which serves only clients (PC, security system, VOIP box, IPTV). Other 2 deco's are wirelessly connected to this master deco.

I've ruled out the cables because I'm able to get regular speeds 500+ mbps through wired and wifi connection (right next to deco) after reconfiguring the network, but after a few minutes my speeds drop to 10-20mbps. Also ruled out my ISP modem because I hardwired my PC to the modem when my Deco speed drops, and am able to get 500+ mbps.

Appreciate any help, thanks!