@wifi6japan 

If the issue was noticed after installing the firmware, have you tried reverting to the previous version to see if the issue stops?

@Tony Yes I tried downgrading but without success. Also I just found that in your US website, there's a warning specifically mentioned that this latest firmware update is irreversible. 

"Note: You will be unable to downgrade to the previous firmware version after updating this firmware." Source link.

@wifi6japan 

Thank you for the correction.

We have not heard of anything recent of Wi-Fi 6, but I am seeing what I could find on that.

In the meantime, have you tried modifying the wireless settings on the 5G? For example, modifying the channel, channel width, and setting security to WPA2-PSK AES?

@Tony Thanks for getting back to me. 
 

I've tried the following approaches. 
 

* Manually switching to a different channel (In fact, when switching back to 802.11ac from 802.11ax, even if it's the same channel, I would see a huge speed and lantency bump)

* Manuall setting channel width to 80

* Setting security to WPA2-PSK AES

* Factory reset and wipe everything including PPPoE credentials

None of the above worked for me.   
 

@Tony 

By the way, I do notice one weird behavior with this new firmware under 802.11ax. When I manually set the mode as 892.11ax and the width as 80Hz. And then had my iPhone 11 Pro Max connected to it. I noticed that my iPhone would always report that it was coonected to a 20 Hz width WiFi, even though in the AX10 router's setting I explicitly set the width to 80. Attached is a screenshot. 

@wifi6japan 

My apologies for the delayed response, the tech team was made aware of your issue.

They will likely reach out to you via email to continue to troubleshoot your issue.

@wifi6japan Experiencing the same issue. Change to 2.4Ghz and everthing is fine. Go to 5Ghz channel and connection is solid but throughput to router is unusable.

Firmware Version:

1.1.0 Build 20200908 Rel. 42617(5553)

Hardware Version:

Archer AX10 v1.20

Due to the slow responsiveness of this thread, and the unacceptable fact that I am now allowed to downgrade to the older firmware, I am now taking actions on my own. Here you can see my 1-start review in Amazon Japan.

@wifi6japan I am having the same issue (sadly, this is when I connect to either band). I just got the router about an hour ago, hooked it up and ran a speed test. I was hitting 190 down and 186 up. I did the firmware update and now I am lucky to hit 70 down and I get around 140 up. This is terrible. Does TP-Link not test before pushing something out? And why would they make the firmware irreversible? My AmpliFi router at least got 80-90 down and 190 up. The entire point of purchasing this device was to get better speeds, not worse than my previous generation. I will wait until tomorrow to see if someone posts something useful, if not, this device is going back to Amazon!

@GregoryF 

Go to advanced wireless settings and at the top, disable OFDMA. That has fixed my issue. Also check if your wireless card has a driver update.