I was able to add a USB adaptor on Shadow, under 3.x firmware, and even put in a request for limited support for doing this, but it went nowhere.
https://forum.gl-inet.com/t/support-for-usb-wifi-adaptors/13881?u=eric
The problem is GL iNet firmware is normally based on an old version of OpenWrt, and is tied very tightly to their hardware, so if you add anything it does not expect, and get it to work, it normally breaks on the next incremental firmware update.
If you want to do this, I would move to generic Openwrt, and give up on GL iNet. The RTL8821AU chipset is supported in OpenWrt 25.12