After more fiddling with the router, I was able to connect to the router with 2400 Mbps speed. I had to change the channel to 120 (DFS) and then I could see link speed go past 1200 Mbps, with clear line of site it was 2400 Mbps.

However, all AC device which was connected got disconnected, maybe because they don't support that channel or router did run in 160 MHz mode in that channel and AC device doesn't support 160 MHz. That made 5GHz band to disappear from the phone's wifi scan list. My theory is, even if I set 160 MHz in the router config page, if the channel is not DFS or out of AC band, the router run in 80 MHz mode. This also means that if we want to use AC and AX device, the maximum bandwidth won't be 2400 Mbps but 1200 Mbps. So this router is backward compatible but it doesn't give claimed bandwith of 2400 Mbps when AC and AX devices.

After one hour, the router automatically changed the channel from 120 to 149 and nwo the 5 GHz appeared again to phone wifi list and I could connect to it. The laptop's intel AX200NGW could achieve a maximum link speed of 1200 Mbps.  This automatic behaviour is either a bug or intentional, either way it is unacceptable as I have no control to a device. It is unfortunate that this is the second iteration after AX15 of AX3000, but still we don't have a bug free version of the device.

Further I tested the USB 3.0 speed by connecting a Lexar Triton USB 3 with maximum speed 170 read and 170 write speed. I got no more than 15 MBps in write and 25 MBps in read when router was connected with 2400 Mbps speed. The result of these transfer is again disappointing .

In view of my observations since last 2 days, I have decided to return this router if seller allows it. I should look for another model or another company.

I have had similar issues with connecting to the 5G band from multiple devices while on 160mhz.  Anywhere from "Authenication Error" to can't connect to network and when it did connect took up to 30-60 seconds (but connections were rare), even though the connection had been established and working fine without any changes minutes earlier.  Another problem was the 5G SSID's disappearing fom the available wifi list on all devices.

I got into a chat session with a tech support agent who eventually suggested that I set the channel width to 44mhz on both the 2G and 5G bands, save the settings and unplug the router for "3-5 minutes".  This completely worked with no further connection problems  but not surprisingly screwed up 5G wifi speeds dramatically. I ended up going to 80MHZ on the 5G band that has worked ok.  After a while I went back to 160mhz and the connection problems returned immediately.

Obviously resorting to 50% of the bandwidth to be able to connect is a no-go and NOT why I purchased an WiFi 6 router.  So far I have one desktop with a WiFi 6 PCIe adapter and that makes no difference with the ability to connect or not.  To me this means the AX3000 cannot handle consistent connections at the 160mhz channel width and will unfortunately be going back to the seller to exchange for a different brand.

@Kevin_Z Actually, the problem is not about negotiation speed but inherent problem of the device and false marketing. This router do connect at 2.4 Gbps in pure AX mode but then it does not connect to any AC device. If both device need to be connected and supported by this router, it has to run in 1.2 Gbps mode. So people like us, who has mixed collection of AX and AC device, this router doesn't give advertized 2.4Gbps bandwith but 1.2Gbps and which 50% less than what has been promised by TP Link. It is not mentioned by TP link that this will be the case, instead TP Link advertized that it has 2.4Gbps bandwith and it is backward compatible with AC and N devices which is misleading and create confusion and frustration among user. TP link should be clear and say that this device will be backward compatible and will provide a maximum of 1.2 Gbps, 2.4 Gbps is also achieveble in pure AX mode which will not support AC device or TP link should provide a triband router with different bandwith specified in each mode.

i had similar issue as well. the tech support isnt able to resolve my issue. i am using deco x60 which isnt able to manually set to 160mhz. its at 80mhz hence my archer tx3000e wifi adapter can only be running with link speed at 1.2gbps instead of 2.4gbps. regret getting tp link not knowing that the software customisation is so basic and not able to run at its full potential 

@mrspid3r I I long ago gave up on the AX3000 and switched to the TP-Link AX11000.  Yes, it costs more but it has been working excpetionally for me on all of my mixed devices and at full speeds.  I'm getting 150+ mbps on non-AX wireless devices and 300+ mbps on AX devices.  My ISP speed is 600.  The AX11000 is BY FAR a better router with a hugely better UI and feature set for device and overall management.

I can get 800mbps on my AX50 with AX devices. That doesn't change the fact that it's a bad router plagued with bugs. I wouldn't recommend the AX11000, nor any other AX based products from TP-Link, just because it can transmit 300mbps... Which is as slow as a midrange AC router...

@NitinKumar 

Yeah. I think you are right. When I was testing I managed to get it to work at 2.4Gbps. The moment I connected my smartphone, which only supports AC, it failed to connect the first time. Then it auto connected... and my AX200 was working again at 1.2Gbps. This router does not support 2.4Gbps. Period. Suppose you upgrade all your devices to AX to have it working at 2.4Gbps... it will fail the moment you have a guest in your house without an AX phone :P I guess it will work at 2.4Gbps during the pandemic. Lol.