@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

@Rhino30 

@TP-Link

I thought there was something wrong with my X20 since I have done a lot of troubleshooting together with TP-link support in the recent weeks. But after reading this post, I understand that we are many with the same problem. I expanded my M9 network with an X20-4G in the belief of making an upgrade but now it poses more hassle than benefit. Previously I had a separate 4G router that I switched manually and it turns out to be a better solution than this!

I completely agree that using another Deco is not a solution. First, I want stable wired connection to surveillance cameras, alarms, TVs, smart home gateways and more. I have all my Decos connected by cable to get fast internet throughout the house. If I should connect my wired devices to a satellite deco, it would not solve my problem because it would not be able to communicate with my wired deco in the basement, for example. I really hope TP-link rethinks and makes it possible to dedicate one of the ports to WAN so that the failover works as expected.

Has anyone else got the beta version installed? And why not release a firmware update if there is a working solution? The existing one appears to have been released on March 29, 2021.

Would be awesome:

Having an option in the webUI/Deco app to turn on WAN on the specific LAN port at the bottom of the device

Then showing it clear in the webUI/Deco app as an option with a little drawing showing where to put the cable from the internet and the one for your router/device if you enable/disable this setting (drawing thinking about the not technically inclined people)

Just ideas ;)

@TP-Link
I got the same issue in deco M5
I have cable to a device on LAN connection, and a cable to a ISP Router.
When I lost internet connection, or disconnect the ISP Router I lost connection with the LAN device.

but Wireless devices are connected ,only LAN is disconnected..

  @TP-Link With due respect... This is not a solution for such an expensive product.