Hello and I hope that you are both safe and well. In order to flash to OpenWRT and back to R7800 stock easily - just follow these steps below Step 1 comes after Step 3 for obvious reasons :
Download all needed firmware and application ( s ) before beginning
1 - Set static ip address on your computer - I use Windows - see below
To set a static IP address in Windows 7, 8, and 10 Set the following address below :
Select Use the following IP address and set the IP address information like below: IP address => 192.168.1.10 Subnet mask => 255.255.255.0 Default Gateway => 192.168.1.1
2 - Download and save the router’s firmware into a folder on the desktop of your computer.
3 - Download latest Tftpd64 ( v4.64 28th Feb 2019 ) - I use the portable version from here Tftpd64
4 - Then follow the instructions from this page : How to upload firmware to a NETGEAR router using TFTP client
5 - As I said this works with both OpenWRT factory images and NETGEAR OEM Firmware - and never sysupgrade .bin files - these are flashed via Luci while running OpenWRT.
6 - Latest NETGEAR OEM R7800 Firmware download is here : R7800 — Nighthawk X4S AC2600 Firmware Version 184.108.40.206
That's about it and I hope this helps
Peace and Stay Well
Video Gives You An Example Here : TP Link Restore to OEM Factory Firmware
Even though this video uses a TP Link Router the process also works with NETGEAR R7800