Well unfortunately it was on 5g and even on lan its speed is slow 

speedtest webpage data ?

hi, did you check the wired/wireless link speed of your device, which will affect the actual speed. TP-link released an intruction to guide you to do that. 

https://www.tp-link.com/en/faq-2265.html 

Meanwhile, would you post a picture of the speed test from 2 different routers to do a comparison analysis? 

Or follow the instruction below to narrow down the issue. https://www.tp-link.com/en/faq-2170.html 

one more thing, you can change another ethernet cable with better quality to give it a shot

AC5400x (lan) vs AC1200 (lan) 

300mbps vs 720mbps

wires are the same (D link cat 6)

yea its on the latest firmware but I heard there is a firmware that is currently on beta but that fixes the issue but its not released yet

TP-Link have a new firmware for Archer C5400X which haven't been released yet.

You can try to upgrade your router to it and check whether the problem can be solved.

Here is the download link: http://support.tp-link.com/f/165f0aeda2/

here the error persists even after the beta update a penalty.