@Python 

Hello, we will get some details to do further analysis and try to fix it. 

What is the current network topology/layout? And may I know who is your internet service provider? And what's the bandwidth you pay for them?

With your device connected to the modem directly, what is the actual speed you can get separately? And please click here to check the wired negotiation speed.
Note: The steps are the same, while you need right click on local area connection or ethernet connection.          

                                                                
Besides, please try to run a speed test via the Deco app and show us the results. 

With your computer or cellphone connected to the main Deco only, with other slave Decos unplugged, please run the speed test 3 times and show us the test results; Furthermore, please connect your computer to the slave Deco by ethernet cable to verify the wired speed.

You can swap the locations of the Deco units or change another slave Deco to the main one. 

After that, please run the speed test again and check whether it works.

Have a great day.