@Razschwe 

If you don't want to touch Deco Main Network, enable Guest Network, make it 5GHz only and run speed tests with device connected to Guest Network.

Also, before running speed tests on device check its link speed:  How to check the associated/link speed on a wireless client

You should see 866Mbps if your device uses WiFi5 and more than 900Mbps it if uses WiFi6.

  @Alexandre. 

Thank you for the good idea's.

I tested Guest network with only 5ghz - No help, same speed, I get 75-90mbps downloads and 95~ upload

Checked link speed as in the instructions - I get 432Mbps in my laptop connecter with a Wifi6 network card.

  @Razschwe 

432Mbps link speed is too low even for WiFi5. Do you have any other device, such as smartphone, to use to check link speed on it? That would help to narrow the issue to either laptop or Deco.

For WiFi5 you should expect link speed of 866Mbps, and for WiFi6 it could be as high as 1200Mbps.

Also, this FAQ document may be helpful and it has links to other helpful documents: Troubleshooting: Slow Internet Speed with Deco Devices

  @Alexandre. 

Thank you,

After some more investigation I found out the cable was sometimes not working well.

Tested it with a tester and there were missing signals when moving it around.

Replaced the cable and it's back working fine now.