@Antycher
Hi, for this issue, we would like to do some troubleshooting to check the issue first.
Please follow the instruction to do some tests and try the suggestions. https://www.tp-link.com/support/faq/2843/
If it doesn't help, please tell me what troubleshooting steps you have done in detail and the results.
Meanwhile, please provide the screenshots of wired and wireless speed of the router, the speed on your phone with the SIM card.
BTW, may I know when and where you bought the router?
Look forward to your reply.