@Kronborg 

There are simple troubleshooting steps you could try. They will be focused on Main Deco, because Satellite will never deliver faster WiFi speeds than Main.

Here they are:

1. Power off Satellite Deco. This is to make sure your mobile device is connected to Main Deco.

2. Turn off 2.4GHz band in Deco app for Main Network:

3. Use OOkla Speedtest app to run speed tests. It is available for iPhone and Android.

4. Make sure VPN is disabled on mobile device you use for speed tests.

5. Make sure mobile device you run speed test on is connected to Deco WiFi. Place mobile device in direct view of Main Deco, connected to Deco WiFi, at the distance of 1-2 meters (3-6 feet) from Main Deco.

Run multiple speed tests in that configuration. From M4, you should expect at least 500Mbps on gigabit Internet and with little WiFi interference from environment.

After running speed tests you can undo changes and power back on Satellite Deco.

If these make no difference, I'll have one more step for you to try.

  @Alexandre. 

Did everything you suggested and speed is just above 500 - not impressing. I left it on 5Ghz and have just tested Deco Wifi and Router Wifi.

Deco: 175/65

Router: 272/103

  @Kronborg 

There is one more step you can try. 

Configure Deco mesh to run in Access Point mode: Switching a Deco to Access Point (AP) Mode

This will disable all settings in Deco mesh responsible for traffic management: content filtering, bandwidth throttling, etc.

Then, repeat steps 1 through 5 from my original post, i.e., repeat all steps I listed but not with Deco mesh running in Access Point mode.

Run speed tests in this configuration. Let me know results you've got.

  @Alexandre. 

It is allready in Access Point mode...

  @Kronborg 

Running Deco mesh in AP mode means it delivers the best it can in your environment. 

I can't tell why M4 speeds you see are that low. In general, one could expect at least 500Mbps from Main Deco M4, perhaps a bit more. I am getting up to 650Mbps on similar Deco WiFi5 capable hardware, with tests done over gigabit Internet link in a place with extremely low WiFi interference. 

Your Arris router should be at least WiFi5. The fact you can't get at least 500Mbps from it, too, points at something common between it and Deco. Either the way you run speed tests, or something with environment in which you placed Arris router and Main Deco. Without in person visit to your place nobody can help you with figuring what's up.

Here is frank assesment of situation. Put aside Deco mesh. If you can return it to retailer - send it back. If Arris router is yours and not from your ISP - contact your ISP and request their router.

If Arris router provided by your ISP - contact them and ask for WiFi troubleshooting. They may send their technician to your place and after technician figures why can't you get decent WiFi speeds from their router - you decide if you want to add WiFi mesh to it.

  @Alexandre. 

Thanx for your time