@DanielSweet 

Hi, 

In your case, please connect a laptop to the upstairs Deco by an Ethernet cable directly, and turn off the Wi-Fi on the laptop, can you get internet access on the laptop? If yes, it means that the wired connection from the Deco is no problem.

Then you can connect the switch back to the upstairs Deco, and then connect the laptop to the switch by an Ethernet cable direct, and test if the switch is working.

If yes, you may need to check if your computer has configured some settings such as static IP thus it can't go to the internet properly.
If not, it may be the switch has an issue.

@DanielSweet Were you able to solve this issue? I am experiencing the same thing. Thanks.

@DanielSweet 
Same issue wifi working but Lan wire says good bye in occasions 

@TP-Link_Deco you need to research more on this and come with conclusive resolution it is widespread issue and reported in many forums 

@DanielSweet I have exactly the same issue

@Samsilv @BabyPanda 

Hi, sorry for the delay.

Since many users echoed with the same issue but did not provide enough details, it was quite hard to finally locate the problem;

If possible, could you please refer to the following suggestions:

1. What are the model number as well as the firmware version? Was it also Deco M5 with firmware 5.3?
2. Can I have a picture of the network topology, such as:
3. When the Ethernet connection failed to get internet service, what was the IP address on these devices?--how to check the IP settings (https://www.tp-link.com/support/faq/841/)--a picture of it would be highly appreciated;
4. In case it was the cable& port issue, please try to switch the port and change a new cable;

 After that, you could leave me a private message for notice;

Thank you very much.

@tplink 

I am having a similar issue.

I have TP-link deco E4 - AC1200 whole home mesh wi-fi system (2-pack)

I have them set up and they work well wirelessly. I recently ran an ethernet cable from my lounge (main Deco Unit) to the Deco unit in my bedroom to try create an ethernet backhaul, as soon as I plug it in my blue TP link light on the top of the unit goes red and I lose internet on all units.

I have tried through a switch and directly to the main deco and i have the same issue every time.

I have followed the config below with no success. 

Please help

@TP-Link hi I have the same issue my Linux is connected to my M5 via ethernet and even though its connected to the Internet but I cannot access it via another device I.e. I cannot ssh into it.

When I reboot the deco m5 which my Linux is connected to I would be able to gain access fine but then after a few hours it would be unaccessible again.

Can you please help? It's really annoying!