@rmason 

I think I may be able to explain what is going on and offer configuration settings you could try, to address the issue. Also, I can provide an alternative ironclad solution (but you'll have to spend few extra dollars on it).

To not make that post really long, I may reply later with separate post discussing what is this new feature "locking device to a single Deco," which is available in firmware 1.6 for Deco M5 only (currently), and why it may not be solution for your problem even when it comes with some future firmware upgrade for X20s you have.

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What is going on.

I will start with the assumption you have modern WiFi adapter. I googled TP-Link Archer TXe75e and I can tell it is. This adapter should know what WiFi mesh is and how to use it. You also have modern Deco WiFi mesh system from TP-Link.

I make that important point, because you may have seen many complains on that forum from people using old devices such as security cameras, which run on 2.4GHz only, and these devices obviously don't understand what WiFi mesh is and why there could be multiple nodes broadcasting same SSID. They just connect to whatever WiFi mesh node they find first and stick to it forever.

Yours is not that case, so ignore those forum posts as irrelevant to what you have.

Your PC connects to Deco 1, occasionally switching to Deco 3. The only reason I can think of that behavior is some sort of brief WiFi interference appearing between PC and Deco 1, to the point when PC realizes it can't talk reliably to Deco 1. Hence, it looks for different WiFi mesh node it can talk to, finds Deco 3 and connects to it. This is the proper functioning of WiFi mesh and wireless client, if root cause is WiFi interference.

That WiFi interference could be something obvious, such as microwave or cordless phone in the room with Deco 1, or PC, or in adjacent rooms. Could be less obvious such as high power electrical cable going in the wall between PC and Deco 1 rooms, to electric stove or electric dryer. Could also be something out of your control, such as your neighbour playing with WiFi jammer or nearby military installation testing radar equipment.

If you can figure source of WiFi interference, and it is under your control, take care of it. 

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Configuration settings.

If root cause is WiFi interference, especially not under your control, the solution should focus not on how force PC always stay connected to Deco 1. Besides, you tried that and it did not work. 

The solution and configuration settings will focus on how to force PC come back from Deco 3 to Deco 1 as soon as possible, with no interruption or disconnect.

For that, please do the following configuration changes all at the same time:

1. Set Roaming Aggressiveness to Highest in WiFi adapter's advanced settings.

This will force PC rescan WiFi mesh nodes as frequently as possible and make PC connect back to Deco 1 as soon as it can. Keeping roaming at Lowest forces PC to stay with Deco 3 until it loses signal. Only then it reconnects back to Deco 1, and that's why you see brief loss of Internet connectivity on PC.

2. Turn on Mesh for desktop in the Deco App

The point of that feature is to have Deco mesh tell PC which is the best Deco node it should be connected to. When you turn it off, PC will keep staying connected to Deco 3, because WiFi mesh is not going to tell PC reconnect to Deco 1 with better signal.

3. Keep the Preferred Band as 5GHz in WiFi adapter's advanced settings

This was the correct configuration setting you've made, but notice "Preferred." I have such network card, and "preferred" means what it means. I also have different network card where I can specify "5GHz only." I like that other network card more.

Your TP-Link Archer TXe75e adapter will try connecting on 5GHz, but if for any reason it finds connection not so good, it could abandon 5GHz and use 2.4GHz instead. Which is not desirable outcome. Luckily, you can force PC always use WiFi5, in Deco app.

4. Make sure Fast Roaming is Enabled in Deco app.

You can find it under More/Advanced.

5. Turn 2.4GHz band off for Main Network, in Deco app

This will force PC connect on 5GHz only, and also will make PC less likely to consider Deco 2 and Deco 3, because it'll see extremely weak or non-existent 5GHz WiFi signal from them, and won't be allowed to use stronger 2.4GHz instead. 

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Alternative solution.

I am assuming your PC has Ethernet port. An alternative solution would be to get one more Deco X20, place it side by side with PC, wire PC to that X20 by Ethernet cable. 

  @Alexandre. 

Thank you for the extremely detailed reply. I tried most of those and still was seeing occasional switched from Deco 1 to Deco 3, although not quite as frequently. What did actually work was just moving Deco 3 from being out in the open to putting it in a closet lol. That seemed to weaken the signal enough that now I havent seen any disconnects from Deco 1 since doing so, and Deco 3 is still able to cover the area it really needs to. Thanks for all the tips!

  @Alexandre. Noticed you said " firmware 1.6 for Deco M5 only (currently)."  I could really use this feature, but I have the Dexo x3600 and the latest firmware update from a month or so ago didn't include any new features.  I've been browsing the forums and it seems like this has been requested by people for 3 years.  Any idea if they're planning to roll it out to more devices?

  @R5Ryder 

TP-Link Support now offers beta firmware 1.6 for M9 Plus, in addition to M5. They promised firmware with that feature for some other models of Deco, but ETA is not clear. I don't know what problem you are trying to solve with this feature, but I think it is time for me to fulfill my promise and explain "why it may not be solution for your problem even when it comes with some future firmware upgrade for Deco model you have."

Most people on that forum asking for "locking device to a single Deco" are complaining their security cameras and other smart devices do not connect to the nearest Deco. The common thing between most of these devices is they run on WiFi 2.4GHz only. These people will be disappointed when firmware with "customizing clients connection preference" feature is released and they try it.

WiFi5 was adopted in 2014. If wireless device does not support WiFi5 and only runs on 2.4GHz, that means device WiFi connectivity code was written more than 8 years ago. Household WiFi mesh was not a common occurrence 8 years ago. Software developers who wrote code for wireless devices followed realities of that time: single household WiFi router. You give that security camera SSID/password, it scans environment, finds first WiFi node that accepts SSID/password and connects to it. Strong WiFi signal or weak, does not matter - wireless device assumes, there is single router broadcasting with that SSID/password. Device does not look for anything else. It finds Deco node randomly, by its scanning algorithms, and what it finds might not be the nearest Deco.

Deco mesh, just like any other WiFi mesh, uses mesh protocols to tell wireless devices if there is better WiFi mesh node to connect to. It is "Mesh" toggle at wireless device name in Deco app: leave it enabled and Deco mesh will automatically direct device to the Deco node with stronger WiFi signal.

The problem is, these mesh protocols are not supported by older wireless devices that don't know WiFi mesh exists. Deco mesh recommendations will be ignored.

So, what is really that new feature in firmware 1.6 for M5 and M9 Plus that "customizes client connection preference?" It is the same mesh protocol Deco uses to tell wireless device which node connect to. Except, instead of choosing best node automatically, it'll take your input. You will be telling Deco mesh "please, instruct that wireless device to connect to this Deco node." What will security camera do? It'll ignore it.

For WiFi mesh to work properly with wireless clients, you must have wireless clients supporting WiFi mesh.

The proper solution is hardware upgrade: replacing security cameras and other IoT devices with newer models, those that at least support WiFi5. If you find one supporting WiFi6, which was adopted in 2019, I'll tell you with utmost confidence such wireless device will know what WiFi mesh is.

With time, the situation will improve and wireless device manufacturers will all support WiFi mesh in their new products. Now that household WiFi mesh becomes common occurrence, if someone has it at their home and buys new wireless security camera which can't figure best WiFi node to connect to, that camera will be sent back to the manufacturer.