Wireless USB adapters used presently are Buffalo WI-U2-433DM that would have the rtl8812au specification, and Chinese EDUP with the rtl8811au.
The PC used is FMV-AH550/5B (core i5, memory 4Gbx2, SSD 500Gb). There is no option for a 'secure boot' in the BIOS. The OS is the Linux Mint Wilma Cinnamon ver.22.

I could not set up the wifi on the Q4OS and the Ubuntu 22.04LTS at all, which was unexpectedly troublesome. But as I got to understand more about the linux, I am able to set up the wifi.

Anyway in conclusion, it is found that if the rtl8821au is installed correctly in the linux mint, the rtl8812au works at dual bands (2.4/5GHz).

The procedure to install the drivers is as follows, referring to https://github.com/morrownr/8821au-20210708 [Note],*

$ sudo apt install git
$ git clone https://github.com/morrownr/8821au-20210708
$ cd 8821au-20210708
$ sudo sh install-driver.sh
and,
in case input Y,
cntl+x to quit then,
Y ---> rebooting

After rebooting, either of the USB adapters is plugged in, then the 5GHz band is selected, hence the connection speed is about 300Mbps (upload: 220Mbps).

Note:
*Another version is in
https://github.com/morrownr/8812au-20210820, which can also be used to activate the wifi.