Power cycle your modem, PC and router is always a good start. Connect the PC direct to the modem bypassing the router to ensure the issue is in the router before replacing it. The issue may be your ISP, modem, or in your wiring. If you still have slow speed without the router, check the wiring. Wire connections is something to play close attention to. Best to have a direct line to the modem. Connections should be of good quality, clean, and the nuts snug. Unscrew, take apart, clean, and reattach helps sometimes. Do not have a three way connector going to nothing (no device attached) Also try different top quality ethernet cables (Cat5e or Cat6) from the modem to router and router to the PC.

Hi, it is necessary to check the line quality when the speed gets slow. 

1. Login into the web interface of the router: 

2. Go to status-device info page to check the ADSL parameter. Can refer to the instruction below:

https://www.tp-link.com/en/faq-547.html 

and you can post 2 pictures showing the normal speed/slow speed separately to do a comparison analysis there. lol