@Loo 

It must be your first experience with WiFi mesh system. You have 5000 sq feet house and you bought Deco WiFi mesh for which vendor claims up to 5800 sq feet coverage. Unfortunately, vendor claims for coverage are exaggerated. That is not for TP-Link Deco specifically, but other vendors do that as well. If you press them hard, they'll explain how they can achieve these numbers and while it can be done in lab conditions, no way it is true in real life.

Which means, you may need more hardware to provide good WiFi coverage over the house of such size.

You bought 3-unit Deco X20 set. This is budget model of TP-Link Deco lineup for WiFi6 capable WiFi mesh. For the house of your size you may need different, more powerful, Deco hardware and/or more Deco units (nodes).

What WiFi mesh vendors don't tell in their sales pitch is that WiFi speed you are going to get on Satellite nodes depends a lot on their placement. You may need to spend time and efforts to find the optimal placement for each of your Satellite Deco.

What they also don't tell you explicitly when selling WiFi mesh, is that every Satellite node cuts speed by half. Knowing that, here is how we can align expectations: if your Main Deco on ground floor delivers up to 500Mbps, Satellite Deco on first floor could deliver up to 250Mbps, and Satellite Deco on second floor will deliver under 150Mbps.

Once again, this is not Deco specific, this is limitation of technology. There are ways to improve that somewhat, but it would mean more expensive WiFi mesh hardware or other solutions such as laying Ethernet cable between WiFi mesh nodes.

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For the question "if I connect PC with Ethernet cable to Satellite Deco, will PC get faster speed" the answer is "Yes." Try that. With one caveat: your PC must have gigabit Ethernet card. If it is the model with Fast Ethernet (100Mbps) network card, you can't get more than 90-95Mbps even with PC wired to Deco.

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If you would like me to, I can help you with tuning your Deco mesh. At the end, I might recommend different Deco hardware or Deco placements in your house, or changing configuration settings in Deco app. Or all of the above.

This conversation may take multiple days, as I am not always online and so do you, and because I'll be asking questions and follow up questions and recommending options for you to try.

So, if you want to undertake this process of assessing what you have with Deco mesh and tuning it, here are my initial questions:

1. What is the length of the floor of your house, from one side to another (in ft). If you have significantly different floors, length for each floor.

2. How have you placed your Deco nodes in your house? Example: "Each Deco node is in the middle of their floor, exactly above each other," or "Main Deco on one side of ground floor, Second Deco in the middle of first floor, Third Deco on the other side of third floor." Could be something else - I need to know placement of Deco nodes relative to each other and their relative position on floors of your house. 

3. What mobile device do you use to run speed tests with each Deco: brand and model (example: iPhone 11)?

4. I'd also like you to repeat these tests for each Deco you have, following this process: use same mobile device for each of tests, stay 3-6 feet from the Deco, turn off/on WiFi on mobile device, verify in Deco app that device is connected to Deco you are standing nearby, run multiple speed tests back to back, note best results. Share results with me.