@Stojan 

Who is your internet service provider and what is the current network topology? 

Without any devices connected, please connect one computer to the TL-WRD3600 directly and to test the speed for 3 times and show us the screenshots of the test results.

Note: Please test the wired speed and wireless speed together.

And please click here to get the wired/wireless negotiation speed separately.

Note: The steps are the same, but please click ethernet connection.

Meanwhile, please check the information mentioned above with your device connected to the modem directly so that we can do a comparison analysis

Please ensure that the current firmware is up to date. And change another wired cable to connect the router and modem, like cat 5e or above.

You can refer to the below instruction to narrow down the issue as well.

https://www.tp-link.com/support/faq/2170/

Good day.