@TP-Link I have the exact same setup as described in your posted diagram only using Deco M5 units. Looking thru all of these posts I have yet to see a response from TP-Link as to whether this issue is actively being worked on? Will a firmware update be rolled out to allow us to assign WAN and LAN ports and have them stick after a reboot? If so when will this be rolled out since this has obviously been an issue for quite a while? It seems so logical to resolve these issues by giving users that want it (not default configuration) the option to assign which port is WAN and which one is LAN. I have seen at least one post where TP-Link installed a debug firmware for a user and was able to assign which port is LAN and WAN so obviously this functionality is possible so what is the hold up on rolling this into the firmware?

@TP-Link 

I have the exact same problem using Deco X20. Is there any intention by TP-Link to release an update to address this problem?

@TP-Link 

This solution does not apply. All of my wired equipment cable modem, main deco, patch panel and switch are located in the same place in the house. Because of this connecting the switch to one of the other deco units is not possible. I am sure that I am not the only one with this type of setup. So basically your suggestion would mean I need to place a 2nd deco unit right next to the main deco so I can plug the switch into that one. If this is the solution TP-Link can feel free to send me another deco M5 at no charge since it is rather useless to have two of these units right next to each other just so my wired network will continue to work during an internet outage. I have several wired cameras that should remain operational during an internet outage that go down because of the way these deco ports behave.

TP-Link wrote

@Rhino30 

Hi, You could have a look at the following link:

[Deco] WAN/LAN auto-sensing