Like the commit that jeff quotes, likely you will need to use the Netgear recovery TFTP flashing process to flash the firmware (using the factory image).
Pretty easy advice is e.g in
- https://openwrt.org/toh/netgear/r7800 (debricking section)
- Netgear R7800 exploration (IPQ8065, QCA9984) - #5 by hnyman (message 5)
Are you saying that you have already used the TFTP procedure with 18.06.4 and failed?