@Marvin_S 

Thanks for the reply.  I have gone through your suggestions and still have a missing or unstable 5G wireless network.  The problem seems to be related to the AX6000 rather than the TX20U Plus and when my PC cannot connect to the AX6000 I find that my cellphone also struggles.  If I select Auto and Auto for channel width and channel, I cannot find the 5G network.  I have tried multiple combinations of channel width and count and the only one which is moderately stable is 40 width and channel 44.  Even with this setting I find that I lose connection and my cellphone tends to revert to the 2.4G network.

I also find that the link speed from my PC is faster with 2.4G than 5G when using width 40 and channel 44 for 5G which was never the case before - 5G was always much faster.  

I am really tearing my hair out about the 5G problem with the AX6000 and hope that you can help me to solve this.  The 2.4G network works without any problems.

 I have another cheaper TP-Link router which I use on my home theater, the AX50, which is perfectly stable both on 2.4G and 5G.  It is not a new addition and I have been using it for well over a year - this is not the cause the AX6000 problems which have only been happening for the last few months. I have the latest FW for both the router and USB devices.

Can you suggest the advanced setting I can use for the Archer TX20U Plus in Device Manager/Network Adapters/TP-Link Wireless USB Adapter/Advanced section that could possibly help to give me a more stable connection to the 5G network?

  @JRSur 

I have gone through the ping command to 192.168.0.1 when connected via the 5G network and it works fine with no loss of data and all 4 replies less than 0.4ms