@kukku 

I am assuming your Deco mesh is configured in Router mode. Following recommendations are based on that assumption.

You can run tests to establish if you have too many Decos, or perhaps they are not optimally located. Run next test only if previous was successful.

1. Run "Test Internet Speed" from inside Deco app. It'll run speedtest from inside Main Deco. If you are getting way less than gigabit on that test, there is a problem somewhere between Main Deco and your ISP modem.

2. Turn off all satellite Decos, leaving only Main Deco. Disable 2.4GHz in Deco app WiFi settings for Main network. With smarthpone or tablet in close proximity, about 6ft away, and in plain view, run speed test from it. The speed you'll be getting is the best Deco M4 mesh can deliver.

If you get less than 500 Mbps, check this forum for QoS configuration setting discussions, also see TP-Link article Using QoS feature of Deco.

3. Keeping 2.4GHz turned off in WiFi settings for Deco mesh, turn on one of Satellite M4, preferably closest to Main Deco, and run speedtest from device in close proximity to Satellite M4, connected to it wirelessly. If speeds you are getting are not satisfactory, check signal strength for that Satellite Deco connection to Main Deco, you can find it in Deco app.

4. Continue turning on Satellite Decos, starting with closest to Main Deco, and testing speed for them till you trigger slow speeds again. 

@Alexandre. Thankyou for your reply. Yes, They are configured in router mode.Unfortunately, very strange,  I cannot find any internet speed test icon in my app. I never had it. Im attaching a shot from my deco app.Thankyou

@Alexandre. Thankyou for your reply. Yes, They are configured in router mode.Unfortunately, very strange,  I cannot find any internet speed test icon in my app. I never had it. Im attaching a shot from my deco app.Thankyou

Hi Alexandre. Thankyou for your reply. Yes, They are configured in router mode.Unfortunately, very strange,  I cannot find any internet speed test icon in my app. I never had it. Under Router settings,I have only 5 blue icons. Wifi, Blacklist,update Deco,Advanced and WPS.Thankyou

@kukku 

This is strange. This is where it should be:

I've seen some features dissapearing from my Deco app before, and only fix was to restart the smartphone. Restarting just app won't bring them back.

If you can't get "Test Internet Speed" back - wait till tomorrow, to give TP-Link time to perhaps fix something on their end, and if option is not back tomorrow open new forum topic with "Missing Test Internet Speed option in Deco app" subject line or similar.

@kukku 

I never thought about regional differences, although I can't figure why speedtest could not be allowed or not supported for specific region.

Anyway, I think there is another place in Deco app you can check if you are able to run speedtest from. See "Step 3" from the QoS article I gave you link to. It implies you can run internal speedtest from there.

I run my mesh in Access Point mode, so I can't verify - this is Router only functionality.

On second thought, I run Deco mesh with M5/M9. What if M4 firmware does not support this speedtest feature? Then, you won't see it in Deco app.

Not having it inconvenient. You can skip Step 1 from my list and try Step 2, but if you get too low speed test results on device that is connected to Main Deco with other Decos turned off, it would not be easy to tell if the problem is withing Main Deco (configuration, etc.) or between Main Deco and ISP modem. Step 1 could have eliminated link between Main Deco and modem as a suspect, or have proven it is a problem.

@Alexandre. Thanks a lot for your help.I had a day off today. Ichecked. I do not even have the internet option in the QoS section that you mentioned in your last post, but I noticed that on my list of deco devices, if I press on them individually they automatically give me the download and upload speed in real time. .....  So. I removed all the deco and all cables in modem and I connected my laptop with a cat 6 to check  internet first from my provider and I got an 880mbps from 1g which unfortunately I never get the 1g but is good enough.Thats the maximum you can get in Malta. I connected then the main deco and with a speedtest from my phone browser I got only 220mbps( around 5 feet away from deco). 220 wifi from 880 fibre I see it very low. Having said that , I managed to find a problem. The second deco wasnt well in the range of the main Deco.I noticed that signal from first Deco to second was weak ,only 1 bar and orange. So I removed one from the garage as I had two and put it in between, at least I solved the speed issue upstairs. Im only getting around 90mbps upstairs but better than 20/30 before. Im still confused why My main Deco gives out only average of 220mbps from 880.. and my phone is 5g.I expect at least am minimum 400mbps .

@kukku 

If you only use Deco mesh to deliver strong WiFi signal through you house, consider running it in Access Point mode. That will unlock maximum possible WiFi performance you could get from it.

If you plan to run Deco mesh in Router mode, review the QoS article from link I provided, and set QoS Download Bandwidth value to 1000 - that might improve M4 performance, if this was the bottleneck.