Hi,

The Archer C1900 has the gigabit ports, theoretically, it can handle the 500Mbps bandwidth. 

https://www.tp-link.com/us/home-networking/wifi-router/archer-c1900/#specifications

For the speed issue, you refer to https://www.tp-link.com/support/faq/2170/  to do some troubleshooting first. 

Without any devices connected, please connect one computer to the C1900 directly and go to speedtest to run the test for 3 times and show us the screenshots of the test results.

And please tell me the wired link speed at that time, how to get that? You can refer to the below link.
https://www.tp-link.com/support/faq/2265/

Note: The steps are the same, you only need to right click on ethernet connection or local area connection. 

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

What's the current hardwar and firmware version? 
 

Thanks in advance, and have a nice day.