GL iNet GL AR300M
https://openwrt.org/toh/gl.inet/gl-ar300m
Without a USB Flash drive;
/dev/mtdblock4: /rom (squashfs, Boot Image)
/dev/mtdblock5 /overlay (where all the read / write stuff is stored)
...to mount the above /dev/mtdblock5 (in the instance of changing from a USB Flash Drive designated as /overlay) and access FSTAB
mkdir /tmp/mtd5
mount -t jffs2 /dev/mtdblock5 /tmp/mtd5
nano /tmp/mtd5/upper/etc/config/fstab