I've used WR703N before with openwrt and here's what I do to backup the existing image

BACKUP WR703N firmware
cat /dev/mtd5 > /tmp/firmware.bin

And it is what I use for restoring image

Restore WR703N firmware
sysupgrade -v -n firmware.bin

I did the following command on carambola2 and found the layout of mtd is different on carambola 2 from wr703n.
root@OpenWrt:/tmp# cat /proc/mtd
dev: size erasesize name
mtd0: 00040000 00010000 "u-boot"
mtd1: 00010000 00010000 "u-boot-env"
mtd2: 00200000 00010000 "kernel"
mtd3: 00d90000 00010000 "rootfs"
mtd4: 00b60000 00010000 "rootfs_data"
mtd5: 00010000 00010000 "nvram"
mtd6: 00010000 00010000 "art"
mtd7: 00f90000 00010000 "firmware"

Should I just use mtd7 instead of mtd5?

cat /dev/mtd7 > /tmp/firmware.bin

Also is the restore command the same?

Best,
-pilypala

Yes, you can use mtd7 for firmware backup. The upgrade command is the same (sysupgrade), but you can alter the params to your liking (see sysupgrade --help).

However please note, that if the operating system does any changes to the firmware during the readout process (cat ...), then you may have an inconsistent/corrupt backup file.