Hello,

So you can get 1Gbps from each M4 by wired cable, while no more than 100Mbps from the wireless 5GHz from the 2nd unit, right? 

May I know what is the speed you can get from the main Deco by wirelessly? And who is your ISP and the bandwidth you paid? 

With your device connected to the 5GHz, it is suggested to disable QoS by device, clear the high priority devices and then test the speed again. 

Meanwhile, please check its wireless link speed as the below instruction: 

https://www.tp-link.com/en/faq-2265.html

Good day. 

@osmarul 

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 modem and main Deco by wired cable directly, 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. 

Try to change DNS server under advanced>>IPv4>>Internet connection type and uncheck obtain DNS automatically and change the primary DNS to 8.8.8.8 and click save.

And try to disable fast roaming and set QoS as standard and clear all the high priority rules.

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

Have a great day.