There are a lot of different version on how to tftp a different firmware version to the Netgear R7500 but this way I use here is the most simple for me. I tested this on my R7500 and it works, but this is just a Guide and I take no responsibility if something goes wrong.
What you need:
- the latest Netgear Stock firmware for R7500 found on the Netgear support pages
- a tftp client where I choose pumpKIN as it is the most easy client for me
- a laptop with static IP 192.168.1.3 / 255.255.255.0 GW and DNS 192.168.1.1
- and a bit of time to follow the process
- Configure you PC/Laptop with static IP 192.168.1.3, netmask 255.255.255.0, and DNS and default gateway: 192.168.1.1
- Connect the Laptop / PC to one of the LAN Ports on R7500
- Open the tftp client and choose the image which you downloaded from the Netgear Support site and add the remote IP of the router which should be 192.168.1.1
- Turn off the router and disconnect it from the energy
- press the reset button on the back on the router and keep is pressed
- plugin the Energy from the router and still keep the reset button pressed
- first the router will have the LED in orange blinking and when the lights changes to white color, than you can release the reset button of the router (not sure if the white is blinking also but I guess not)
- In pumpKIN start the transfer of the file and the file should gets copied to the R7500
- Router will upgrade the firmware and restarts automatically after this is done.
When you use this after a messed up revert back from OpenWrt than you could see also the issue that the 5GHz network is missing after reboot. in this case you should follow this steps mentioned here as this is a known issue after revert back to stock firmware.