@nunorcenteio 

What you describe might be general problem with laptop network drivers, a problem with network drivers not supporting mesh technology, laptop not connecting to the nearest Deco unit, laptop switching frequently between different Deco units.

It seems unlikely that both laptop integrated WiFi network card and USB WiFi dongle have network driver issues, but it could be a bad luck. 

I can share recommendations and suggestions to help mitigate these possible issues, and perhaps one of them will make laptop WiFi performance with Deco mesh more tolerable. If you want to follow them, I'd suggest removing TP-Link WiFi dongle and focusing on integrated laptop WiFi network card. Only if nothing works, you could try same with TP-Link WiFi dongle.

But, before I suggest anything, please share more information about your laptop and Deco mesh configuration, specifically:

Laptop brand name and model (for me to check the specs)

Is it Windows laptop (sorry, can't help with Linux or Apple)

Do you move laptop around your house often, or does it stay at the same place most of the time

Do you have WiFi 2.4GHz enabled in Deco mesh for Main Network? If yes, do you have wireless devices that must use 2.4GHz?

Do you have WiFi 5GHz enabled?

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Finally, standard recommendation you'll hear from Tech Support every time you tell them of that type of issue: please check you have latest networking drivers installed. Assuming it is Windows laptop, check Windows Update and View Optional Updates under Windows Updates. You might find new networking drivers there, under Driver Updates section. See example below:

Note, that your WiFi network card will most likely have different name.

Then, visit network card configuration under Device Manager, and see if you can update driver from there. Example:

Finally, visit manufacturer Web site, find support page for your laptop and see if manufacturer released updated networking drivers or overall firmware update package, and use instructions from there to update your laptop.

Getting latest WiFi network card drivers or confirming your laptop already has the latest is the first step in troubleshooting of WiFi issues. 

@Alexandre. , thanks for your reply.

Well, it's a Windows 10 HP notebook but I don't have it right here to know the exact model.

The integrated Wifi network card doesn't support 5Ghz. The laptop sits in the same place.

The Deco have both 2.4Ghz and 5Ghz enabled.

The drivers are updated.

I've bought the TP-Link Wifi dongle because I thought it was something related with the integrated card.

Tomorrow I will try to connect with my own laptop (I have another house with an Archer C6 and OneMesh with Powerlines and repeaters and without problems) to those Deco's and check if the problem persists. If it works, then I know that it's not related with the Decos. Seems a good idea?

@nunorcenteio 

For newer laptop, I would suggest to check if its WiFi network card allows to choose preferred WiFi band, and if yes - select 5GHz. 

That should improve your laptop performance when it is connected to any WiFi router or mesh, not necessarily Deco.