The only way I have been successful using a USB WIFI adapter with a similar GL-AR300M16 router was to give up on the GL iNet firmware and go with OpenWrt 19.07.7 firmware. I am using a Panda PAU08 USB WIFI adapter which was cheap and has very good range. Here is a link to a post I wrote asking for better support of USB WIFI adapters in the GL iNet firmware, that includes pointers to other people who have experienced the same problems when trying to use USB WIFI adapters: Support for USB WIFI Adaptors
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