So... new guy here
Picked up a GL.inet AR750S-Ext earlier this year, and it worked well enough... for a while. Now it's getting increasingly unstable (drops wifi entirely, lucky if it lasts a couple hours) and it was suggested to me to give it a new lease on life by using regular OpenWRT, rather than the vendor-provided version.
After a bit of digging around, and a bit of trial-and-error, I've reset the router and brung up the uboot interface, following the directions here. After some more trial (and mostly error), I ended up downloading a 'clean' version of OpenWRT from GL.inet here, and got it flashed to the device.
So now, I have an old-ish release (18.06.0-rc1 r7090-d2aa3a1b62 ) installed. I thought the next step would be to use the LUCI interface to upgrade using a sysupgrade.bin compatible firmware file. I based that off of information here, here, and finally the ToH: FW Download page for the AR750S.
In theory, it seems like I should be able to upgrade to 19.07.8, using this firmware(http://downloads.openwrt.org/releases/19.07.8/targets/ar71xx/generic/openwrt-19.07.8-ar71xx-generic-gl-ar750s-squashfs-sysupgrade.bin)... but I get a warning message that says: The uploaded image file does not contain a supported format. Make sure that you choose the generic image format for your platform.
Pretty sure that's the generic image for a GL AR750S... it literally says so, right in the file name. So what am I missing here? What do I need to do next?