In contrast to the AR300M-Lite uboot screen, the AR750S uboot screen at either http://192.168.6.1/ or http://192.168.6.1/flashing.html doesn’t appear to have a way to upload a NOR-based image.

Edit: It appears that uploading a sysupgrade.bin file will flash NOR and the router will boot from that image. Is this the intended behavior / usage?

root@GL-AR750S:~# strings /dev/mtd0 | fgrep -i boot | head -n 1
U-Boot 1.1.4-gcf378d80-dirty (Aug 16 2018 - 07:51:15)