@Achilleas 

If you want to add another Deco node to your Deco mesh, you should buy additional M5. Deco X20 will not make much difference for your Deco mesh comparing with M5, but because X20 is on different firmware, adding X20 will disable some very nice features M5 firmware has. Could be the features you may need to improve your Deco mesh performance.

So, if adding more Deco nodes - stick with M5.

Do you need to add another Deco? I can help you answer that question, but I will need you to give me more information about your home network and Deco mesh:

1. What is your Internet speed (from ISP).

2. What is actual WiFi speed you are getting on Main Deco.

3. Second and third floor Decos, are they near stairway?

4. Firmware version of all your Deco M5 is 1.6.1, correct?

5. Open third floor Satellite Deco information page in Deco app and check what Signal Source says there. Example:

If you can make screenshot of your Satellite Deco information page and share it, that would be better. If not, please tell which Deco (Main or second floor) your third floor Deco is connected to and what is the signal quality it reports?

  @Alexandre. 

Thank you so much for your reply.

It is clear, I'll stay with M5. 

My internet speed is 100 optical fiber. My router delivers 96 (WiFi).

1st Deco delivers 86

2nd Deco delivers around 40-45 but for some reason, this can go down to about 10

3rd Deco usually delivers about 10, however, it can go down to 0,... or up to 35+. 

I've included a floor plan with the placement of the Deco, and a screenshot of speed measurements (top 5 in the list are all from 2nd floor Deco). 

Signal source for 3 Deco is the 2nd Deco, as it should be. All Deco are at 1.6.1.

  @Achilleas 

Main Deco M5 can provide WiFi speeds of up to 500Mbps. Which means, if you ever decide to upgrade your Internet speed, you can go up to 500Mbps without need to replace Deco M5 hardware.

Your Satellite Decos screenshot shows that their WiFi backhaul is very weak. That could explain slow speeds and WiFi speed fluctuation: when backhaul is weak, any sudden brief interference (microwave, cordless phone, etc.) could have big impact. 

The idea you have, to bring additional M5 on ground floor closer to stairs could work, but doing that may not be enough. You will need to move that new M5 as close to stairs as possible, and also need to move second floor Deco and third floor Deco as close to stairs as you can.

With Deco M5, you can wall mount them using third party wall mounts. That wall mount will plug into electric outlet, all you need for M5 to be on the wall is an electric outlet. That works when you don't have space to put Deco on flat surface.

I have done that same thing with my Satellite Deco, it is wall mounted right at the stairway with Deco it connects to down one floor, also at stairway. 

To find if moving M5s close to stairs will work, please update this your picture with electric outlets closest to stairway on all three floors:

Then, I'll give you steps to test with Decos you have if that change will bring improvement, and how big that improvement will be. No need to order new M5 and wall mounts before that. If improvement is noticeable and you want to proceed with it as a solution, I'll give you more details about wall mounts.

If WiFi signal does not improve much, we can discuss other options. It may be possible to rely on powerline adapters, but I'd like to keep that discussion on hold until we can figure if just better Deco positioning can work.

Also, next time you reply please attach Decos screenshot in "Show Map" form, that will help me to avoid guessing which Deco is connected to which. 

  @Alexandre. 

Great info Alexandre! 

Through testing, I discovered that moving the 2nd Deco closer to the windows greatly improved the signal from medium (actually very weak) to strong. It seems that WiFi travels better outside the house through the windows than through reinforced concrete slabs. That is why I've also moved the 3rd Deco closer to the windows side and had a small improvement but still quite low. 

I don't have any electrical outlets in the stairways. The closest to the stairways will have a concrete wall between them. 

  @Achilleas 

You have found perfect spot for 1st floor Deco, and it is unfortunate that there are no electric outlets near stairways on the 2nd floor. The plan would have been to place fourth (new) Deco at ground floor stairways and connect 2nd floor Deco to it. That won't work, without electric outlet near the stairway.

What they don't tell in marketing materials for WiFi mesh is you'll need to spend time and efforts to find optimal places for Satellite WiFi mesh nodes. 

What's left is another option: to try linking 1st floor Deco with 2nd floor Deco by pair of powerline adapters. If they establish link, 2nd floor Deco WiFi speed should be as good as 1st floor Deco WiFi speed.

For that, you must have both electric outlets on same circuit breaker. If not, that is more challenging but still could work. You can't move 1st floor Deco from the window, but any electric outlet on 2nd floor should be tested to see which one is best for powerline link.

Not all powerline adapters are compatible with Deco mesh. When 2nd floor Deco connects to compatible powerwall adapter, it will establish Ethernet link and will report it as such in Deco app. Example (using screenshot from older Deco app version):

If they are not compatible, Deco will stay on WiFi backhaul.

For powerline adapters, I recommend to start with TP-Link brand. You need powerline adapters that only deliver Ethernet connectivity (some also have WiFi, avoid those). Also, look for those that have only one Ethernet port per adapter. I have found that adapters with multiple Ethernet ports per unit have greater chance of not being compatible with Deco.

At my house, I successfully tested powerline link for Deco M5 with different powerline brand: ZyXEL PLA5215. I am not sure you could buy that model any more, check that, but if ZyXEL offers newer powerline adapter model it may be worth to try.

In any case, it is important to buy these adapters with return policy. For them to work or not depends on electric wiring at your house and Deco compatibility, both can't be verified until you actually try. If powerline adapters don't work, you should be able to return them and, perhaps, try different brand.

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In summary, at this point my recommendation would be not to buy additional Deco M5, but buy pair of powerline adapters to link 1st floor Satellite Deco with 2nd floor Satellite Deco.

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PS. Just a thought, and might not be for your situation but I saw someone did that so mentioning it: if you could pass Ethernet cable from 1st floor through its window to 2nd floor through its window, and link two Satellite Decos with Ethernet cable, you'll achieve even better results and without need of powerline adapters. Not everyone likes Ethernet cables going outside house walls, but some people don't mind.

  @Alexandre. 

You've been very very helpful, indeed. 

I already had the TP-Link AV500 (my system before I bought the M5), so I've tried to connect it to the M5 network as you said. I had no success though. 

However, during the process, I temporarily relocated the 3rd Deco to another socket, about 1,5m from the 'apparently more suitable' location I had it before.

For no obvious reason, the signal went from 1.5mbps to a whooping 40-45. And stayed there.

The new location has all the 'wrong conditions' (behind a concrete wall, in front of a big radiator, close to H/Y), but still, it works! 

For anyone else reading this thread looking for help, please experiment with Deco placement. It doesn't have to make sense!