use .img via uboot and .tar from luci
if you have problems with stable:
https://downloads.openwrt.org/releases/19.07.2/targets/ar71xx/generic/openwrt-19.07.2-ar71xx-generic-gl-ar750s-squashfs-sysupgrade.bin
start from snapshot factory image:
https://downloads.openwrt.org/snapshots/targets/ath79/nand/openwrt-ath79-nand-glinet_gl-ar750s-nor-nand-squashfs-factory.img
cheers