@Tushant
The 300Mbps is the wireless link speed in PHY layer, while the TL-WR840N has the 100Mbps ports; it can handle the current bandwidth theoretically.
https://www.tp-link.com/support/download/tl-wr840n/#specifications
We will get some details first and try to fix it.
1. What is the speed you can get from the TL-WR840N by wired cable and wireless separately? You can run a speed test for 3 times with computer connected to it.
2. Meanwhile, please verify its wired/wireless negotiation speed as the instruction: https://www.tp-link.com/support/faq/2265/
Note: For wired negotiation speed, it is the same; while you need right click on local area connection or ethernet.
3. What is the current hardware and firmware version of the TL-WR840N?
4. Besides, you can refer to the instruction below to narrow down the issue.
https://www.tp-link.com/support/faq/2170/
Thanks in advance, and have a nice day.