I have spent the last several weeks working at getting a similar setup working with my AR300M16-ext, using a RT3070 based USB WIFI device. The AR300M hardware works fine with a USB WIFI adapter, but the current GL iNet firmware, 3.105, is not setup to handle a second radio, even when using LUCI in the advanced menu. To get my AR300M16 system to work, I have had to load the generic OpenWrt 19.07 firmware and then add extra OpenWrt packages to it to get back most of the GL iNet features I need. It is now stable, using the USB WIFI to attach to a remote AP, and the local internal radio as my local AP.
I recently wrote a post requesting that GL iNet officially support external USB WIFI adapters on their routers:
but there was no response from anyone who works for GL iNet on if they are thinking about adding this support. You may want to do a search for USB WIFI and RT3070 in the forum for pointers to help you get your USB WIFI working on the AR300M.