Hello,
The "cleanest" way to install new firmware is to use the bootloader procedure. However, if the device came with legacy firmware from the factory (<=v6.xx), then the bootloader will only accept the legacy firmware to be flashed.
In this case, you should flash the latest legacy firmware using the bootloader (RUT9XX_R_00.06.09.2), and then from the WebUI, you'll be able to flash the latest firmware (RUT9_R_00.07.04) without keeping the settings.
However, if you're updating the device with two or fewer firmware release gaps, this should not be needed, and the device can be updated with the keep settings option.
Best regards,
DaumantasG