The factory image.
Just like explained in the R7800 device page and here in the forum.
- https://openwrt.org/toh/netgear/r7800 (Debricking section)
- Netgear R7800 exploration (IPQ8065, QCA9984) - #5 by hnyman
Ps. forget about those backups. You want a clean setup. (and it makes no sense to backup the read-only art partition.)