tp-wn823n adapter not found

Any physical or software means to determine the hardware version (v1/v2/v3) of that three-year old TP-Link adapter? Can you see it in System Information : USB?

As I mentioned previously, the current beta driver software is for the V3 device, and if you have older hardware, you may get the menu extra functionality, without any further driver detection of the older hardware.

Kernel drivers are easily out of date, and incompatible with newer operating systems. You cannot find the TP-Link device because its driver is simply not working on Mojave. Without a current, compatible kernel driver, you have a worthless adapter on Mojave.

The author never mentioned what explicit version of Mojave was in use with that beta TP-link software. If we assume that he was on 10.14.3 which came out on Feb 7, then there have been two more updates to Mojave, any one of them could have introduced Apple changes that snafu'd the TP-Link kernel driver. Or not.

The only Mojave driver support is for the V3 hardware release of this product. Still no apparent non-beta release.

Do you at least get the menu extra's menu for the TP-Link device as he shows in his article.

All Macs for many years have had built-in WiFi as standard so the need for an external USB WiFi adapter is unusual.

As VikingOSX says the drivers aka Kexts need to be sufficiently up-to-date to be compatible with your Mac operating system. Sometimes older driver can be got to work by disabling the SIP security protection with the risks that implies.

As you appear to have already found the article listing a beta driver which might well work I will merely list the following thread which discusses some other possible USB adapters to try - https://www.tonymacx86.com/threads/mini-usb-wifi-adapter-compatibility-with-os-x-mojave.260829/