@punchtownparry 

Hello, please help verify some details and we will try to fix it together:

1. May I know who is your ISP? What is the speed you can get when you connect a computer directly to the modem?

2. Without any devices connected, please connect one computer to the AX6000 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/en/faq-2265.html

3. The speed tested on the Tether app might be different from that on your computer, we may think the Tether speed is more like the connection speed between the router and your internet, which would be different from the speed you can get on the computer.

4. Do you enable any QoS setting on the router? Kindly show me the QoS setting page on your AX600, try to disable it then check again.

5. You can refer to the below instruction to narrow down the issue as well.
https://www.tp-link.com/en/faq-2170.html

Good day.