@Alan062 

The option to separate the 2.4GHz and 5GHz will be added in next firmware, for now they use the same network name and cannot be isolated from each other. 

While we can do some troubleshooting regarding to the speed issue:

1. With computer connected to the Deco's wifi network only, please run speed test for 3 times and show us the results. 

2. Please check the wireless negotiation speed of your computer as the following instruction: https://www.tp-link.com/support/faq/2265/

3. Please turn off the slave Decos and connect the computer to the main Deco directly and test the wireless speed. 

4. Besides, please tell us the current network topology, how do you connect these devices together. 

@Alan062 

Thanks for your reply; from the pic attached, the wireless negotiation speed of your mac is 54Mbps only, it seems that it limits the actual speed you get. 

To narrow down and locate the issue, with your computer connected to the main Deco by wired cable, please check the speed you can get; And connect your computer to the modem directly by ethernet cable to test the speed. 

Besides, try to change another wireless device and connect it to the Deco's wifi and test the actual speed and wireless negotiation speed. 

Do you have other router/old router can be used to do a comparison analysis? If possible, please connect it to the modem and check the wired/wireless speed you can get from it. 

Thanks in advance. 

@Alan062 

Thanks for your reply Alan.

Please clear the QoS settings/high priority items if you have set it up.

Then to narrow down the issue, if possible, please borrow a computer with ethernet port and then we can use it to test the wired speed we can get from from the Deco directly. 

Besides, if you use the MAC to connect to the Deco's network, you can click the wifi name connected and press "alt option" key to get the frequency you have connected, whether it is 2.4GHz or 5GHz. 

@Alan062 

Thanks for your cooperation and reply. 

From the wired speed, the M4 should be working properly. I think it is more related to the wireless negotition speed obtained by wireless devices which affect the actual wifi speed. 

With the mac connected to the 5GHz and get the wireless negotiation speed as 585Mbps, did you test the actual wireless speed you can get? 

Meanwhile, have you tried to simplify the network topology by connecting main Deco to the modem only and test the speed yet? What happened?