@Jopie152 

Deco X60 is capable of much more than 300Mbps. 

You tried multiple things, but let's try some of them in an organized order.

1. Please share your Deco mesh network diagram in Map and in List mode. Make screenshots in Deco app and add them to your reply. I'll share sample screenshots at the bottom of this reply, to show what I am asking.

2. Select single device for all speed testing, I would suggest Oneplus 8t. Install Speedtest app from OOkla on it if you used something else for speed testing.

3. Switch Deco mesh to Access Point mode. Turn 2.4GHz off for Main network. For security use WPA2/PSK. Make sure OnePlus 8t is connected to Deco mesh Main Network SSID and to Main Deco. To ensure proper testing, power off Satellite Deco.

4. Unplug Ethernet cable that goes from Main Deco to your home network. This is to ensure you don't have wired device(s) heavily utilizing Internet link while you are running speed tests.

5. Have only OnePlus 8t and nothing else wirelessly connected to Main Deco. Same idea as previous item, but in regard to WiFi devices. If it is too much trouble, you can keep devices such as Smart Plug and Smart Thermostat connected, those usually don't consume much bandwidth. 

To avoid hassle of disconnecting/reconnecting mobile devices, temporarily add them to Deny List and remove from it after you've done your speed tests.

6. Place OnePlus 8t at about 2 meters (6 feet) from Main Deco. Check link speed OnePlus reports: How to check the associated/link speed on a wireless client

Link speed reported should be not less than 900Mbps, which means OnePlus uses WiFi6 to connect. If it is 866Mbps, OnePlus is connected with WiFi5, and 300-400Mbps or less might hint at either plenty of WiFi interference or WiFi 2.4GHz connection.

7. Open Speed Test app and verify it connects to the nearest (geographically) test server for you. If it connected to server faraway, it may hint at VPN connection. Obviously, speeds will not be very good when your Internet traffic goes through VPN. Make sure VPN is off.

With proper nearby test server and with no VPN, run Speed Test app. Make it run multiple times, note best result.

---------------

Screenshot examples for network diagram in Map and in List mode:

  @Alexandre. Ather screenshot on the way. Tomorrow i wil Do test. I have my 8t plus to my daughter. She is not here. 

  @Jopie152 

  @Jopie152 

  @Jopie152 

  @Jopie152 

Jopie152 wrote

  @Alexandre. Ather screenshot on the way. Tomorrow i wil Do test. I have my 8t plus to my daughter. She is not here. 

  @Jopie152 

It is OK. Any other smartphone that has WiFi6 in its hardware specs can do.

  @Jopie152 

Thanks for screenshots, but these are not the right screenshots. I asked for screenshots with your Deco nodes, in Show List view and in Show Map view.

Also, you can add screenshots as images to reply, just like I did. Makes them easier to read from smartphone. Use Image button instead of File button:

  @Alexandre. 

  @Jopie152 

Thanks. These are the right screenshots. Please run tests as I described, when you have time. Make sure Deco mesh runs in Access Point mode: in that mode, all bandwidth throttling settings are disabled and Deco runs at maximum WiFi performance it can deliver.

Few days ago I had conversation with someone who has 150Mbps Internet link. He set one priority device in Deco mesh with 120Mbps bandwidth. Deco mesh left just 30Mbps for the rest of his devices and he had no idea why is that. What he saw is 30Mbps speed test results with any device he tried.

Turning Deco mesh to AP mode would have disabled priority settings and unlocked his Internet speed for any device at home network.

Not saying yours is same case, but explaining why it is important to run Deco mesh performance tests in Access Point mode.