@UncleEddy 

Can you clarify one thing: the wireless speed through problematic Deco, was it also under 10mbps before you increased your Internet speed or was it up to 100mbps?

The point of this question is to find if changing ISP Internet speed have changed that Deco performance for worse. It should not have, but asking for clarity.

@Alexandre. To be honest, I have never checked the wireless speed of that specific Deco before the speed increase.

@UncleEddy 

You tested wired speed through problematic Deco, so we can dismiss Ethernet wiring as an issue. Few other tests I would suggest, to check if there still might be something wrong with your wired network, or with Deco unit itself, or perhaps there is strong WiFi interference in around slow Satellite Deco.

1. Run Deco mesh as pure 5GHz WiFi network

Turn off 2.4GHz on Deco Main Network WiFi, and also on Guest if Guest Network is enabled. This will remove band steering and also force device you use to speed test Deco connect to Deco at 5GHz for sure.

In my tests with my M9 Plus I was not impressed, politely speaking, with what I was getting on 2.4GHz. 

2. Test for issues with wired home network

With 2.4GHz still turned off, power off good Satellite Deco. Leave just two Decos running: problematic Satellite and Main. Run speed test on problematic Satellite Deco again.

Also check that it uses Ethernet backhaul.

3. Test for issues with switch.

This is variation of previous test, but with one more change. Connect problematic Satellite Deco through Ethernet to second port of Main Deco, bypassing switch.

If you have or could get Ethernet network coupler (example: https://www.amazon.ca/Contican-Professional-Ethernet-Splitter-Connector/dp/B083W8JB1Q) you can use same Ethernet cables and that will be really preferred way of testing.

Connect Ethernet cable that comes from Main Deco to switch to one side of network coupler instead, and Ethernet cable that comes to switch from problematic Satellite Deco to another side of network coupler.

4. Test for hardware issue with problematic Satellite Deco and/or WiFi interference

With 2.4GHz still off and good Satellite Deco still powered off, take problematic Satellite Deco to where good Satellite Deco is. Disconnect good Satellite Deco from power and Ethernet cable(s), connect problematic Satellite Deco to them and power it on.

Run speed test in that location.

If problem follows to that new place, you might have defective unit. If problem disappears, it is specific to location and not to Deco mesh or Satellite Deco unit.

To verify without doubt, if problematic Deco suddenly became good in new place, take good Satellite Deco to where was original place of problematic Satellite Deco and run speed test with good Satellite Deco there.

@Alexandre. A very strange situation is happening. I disabled 2.4Ghz and WiFi speed instantly reached around 380mbps. After 2 minutes the connection from the Wifi stopped working while the signal was still strong. I rebooted all deco's, still connecting with the Wifi but it does not load up any webpage. Turned 2.4ghz back on and it started working again, but slow.. After that I disabled 2.4ghz again and boom it stopped working again.

This is for all my devices..

@UncleEddy 

I absolutely did not expect that. I wonder if you have defective Deco set. Under normal circumstances, Deco should run just fine with 5GHz only, and this is how I run my Deco mesh with M9 Plus.

Here are two more tests I would recommend, first is for workaround but if it works just keep it. Second is to establish with absolute certainty if something is wrong with your Deco set.

1. In this test, set your Deco WiFi same as mine which works just fine for my mesh with M9 Plus. That would be enabling Guest Network and running Main Network exclusively on 5GHz and Guest Network exclusively on 2.4GHz:

Make sure you set password for Guest Network, as by default it is without password.

If by some miracle that works, just keep it. Maybe next firmware upgrade will fix whatever quirk was triggered in your Deco mesh by who knows what.

2. If above failed, bring both Satellite Deco to where Main Deco is, daisy chain them by short Ethernet cables of good quality, with no switches in between. Just like that:

Then, turn off 2.4GHz in Deco app and see if Deco mesh will go down in 2-3 minutes. If it does, that proves with almost absolute certainty that the problem is withing Deco units and/or Deco firmware. 

This is the simplest Deco setup and it should always work and stay stable, if everything is right with Deco.

@Alexandre. So far tried option 1 with some interesting results. Guest network did not even allow me to connect.

I then started trial & error with firmware 1.5.1 and 1.5.3.
 

1.5.3 stopped my entire wifi connection. I could still connect, but Wifi did not sent any network through.

I reverted to 1.5.1 (I was on 1.5.0 and did not give me any update notification so I had to do it manually). 1.5.1 works now with the same slow speeds as before. I need to wait until everyone is asleep so I can continue trying everything out. I will then try turning off 2.4ghz again.

@UncleEddy 

Follow up question: is it too late to return M9 set? It is good mesh (when it works), I have it and I will keep mine.

Yet, you are not using best M9 feature, which is dedicated wireless backhaul. 

You have every Deco wired, so if you can return M9 I would recommend Deco X20 3-units set nstead. About same price as M9 3-units set. When wired, X20 will do as well as M9 (when everything works) on WiFi5, and X20 support WiFi6, which means even faster speeds on WiFi 6 capable devices. 

@Alexandre.  Trading it in for a different model will be difficult/impossible. Swapping the unit for a new pair is though. 
 

What is the exact difference between a X20 and a M9? I read it's both mesh and with wireless functionality.

@UncleEddy 

 

What is the exact difference between a X20 and a M9? I read it's both mesh and with wireless functionality.

Deco M9 is superior when Decos communicate with each other by WiFi. It is tri-band, meaning dedicated 5GHz channel for communication between Deco units. That dramatically improves speed of devices connected to Satellite Deco which communicates wirelessly with Main Deco. Deco M9 is top of the line Deco from M-series (M4,M5,M9).

You are not using this feature, your Deco M9 are all wired by Ethernet. Which is the best possible way to deploy Deco mesh, but M9 won't shine in that setup.

Deco M9 only supports WiFi5, which means you won't be getting speeds faster than 500-600Mbps.

Deco X20 does not have strong wireless communication between units. Deco X20 is budget model from X-series (X20,X60,X90). For when all Deco units are connected with Ethernet cable, like in your case, X20 should perform as well as M9 or better. 

Deco X20 supports WiFi6, which means, at least theoretically, speeds up to gigabit on wireless devices that also support WiFi6 and are connected to X20.

Why did I choose M9 over X20:

- Form factor: I needed to wall mount one of Deco units in narrow corridor, tower units won't do there.

- Dedicated WiFi channel to communicate between units: one of my Satellite M9 is wirelessly connected to Main M9.

- No need for WiFi6: in my household, devices using WiFi are smartphones and tablets. 400-600Mbps is more than enough for them. All serious equipment, such as servers, professional desktops, etc. - they are connected by Ethernet to my gigabit home network.