You could try the snapshot if you’re up for it😄

https://dl.gl-inet.com/?model=e750&type=snapshot

However I have no confirmation if power save is supported there. If it’s not & you run into stability issues you can always downgrade. Just be sure to make a backup in Luci before upgrading then

~Edit: There is also an OpenWRT image available, you’ll have to flash through uboot: https://downloads.openwrt.org/releases/21.02.3/targets/ath79/nand/openwrt-21.02.3-ath79-nand-glinet_gl-e750-squashfs-factory.img

However this does require some knowledge about OpenWRT & you will no longer have the GL-inet admin panel.

You can find more information about this here: https://openwrt.org/toh/gl.inet/gl-e750