Hi,

What is the model number of the router? 

Below is the instruction to narrow dow the root cause.

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

Meanwhile please tell us some details:

1. the picture of the speed test with your device connected to the router directly.

2. the wired/wireless link speed of your device, instruction is https://www.tp-link.com/en/support/faq/2265/

Besides, you can change some wireless settings of router, like wireless channe and channel width to give it a go.

Hope it works.