To find the correct version of OpenWrt firmware for my GL iNet routers, I used the OpenWrt firmware finder: https://firmware-selector.openwrt.org/

For the GL-AR300m16, here is the link for the 21.02-rc1 firmware
https://downloads.openwrt.org/releases/21.02.0-rc1/targets/ath79/generic/openwrt-21.02.0-rc1-ath79-generic-glinet_gl-ar300m16-squashfs-sysupgrade.bin

If you would like to try the released OpenWrt 19.07.7 code, it is at:
https://downloads.openwrt.org/releases/19.07.7/targets/ar71xx/generic/openwrt-19.07.7-ar71xx-generic-gl-ar300m-squashfs-sysupgrade.bin

To load this firmware on my AR300M16 running the GL iNet 3.201 firmware, I first upgraded to OpenWrt 19.07.7 by scp the bin file to the /tmp directory on my router, which is a RAM based file system, so it does not use any FLASH. I then ran the sysupgrade command from the AR300M16 router:

# sysupgrade -v -n /tmp/openwrt-19.07.7-ar71xx-generic-gl-ar300m-squashfs-sysupgrade.bin

After the router rebooted, it comes up on IP address 192.168.1.1 running OpenWrt 19.07.7 without a root password. I then did the same thing again, using the 21.02 bin file and sysupgrade, and after the reboot, the system was running 21.02rc1. I did not need to use uboot.

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