@Bikas 

Hello, sorry about that, we have added the AX90 to the community now.

For the slow speed issue you reported, please help verify some details and we will try to fix it together:

1. May I know who is your ISP? Is the ~900Mbps Download speed tested when one computer is connected to the AX90 LAN port or to the modem LAN port? What is the actual download speed provided by your ISP?

2. Do you enable any QoS setting on the AX90? Kindly show me the QoS setting page, ensure the QoS settings on the router is on Standard mode, and no high priority devices, if the speed on the page doesn't display correctly, change it to Set Manually and type in the correct bandwidth provided by your ISP. Or simply reset the AX90 and reconfigure it from scratch, leave all settings by default and test the speed again.

3. What kind of devices are connected to the 2.4GHz wireless and what is connected to the 5GHz-1 and 5GHz-2? How about the link speed on them? If it is a computer, you may check it like below:
https://www.tp-link.com/support/faq/2265/
Note: If the link speed is not as good as expected, please try with another computer with a more powerful wireless adapter. BTW, tell us the model number of that wireless adapter that runs on your computer.

Good day.

Get a wifi scanning app to see if there's any other 5ghz routers near the channel you's is using.  You may just have to manually change to a vacant channel farther from the channel your neighbor's router is on. @Kevin_Z