Hello,
The newer RUT240 devices are designed to only work with the latest RutOS versions (v7.0>), thus legacy firmware cannot be installed on them using the bootloader. In this case, the bootloader is too new and does not accept your custom firmware. You might need to recompile the SDK with your changes on the latest firmware or install the firmware via the WebUI.
My colleagues will provide more information on the Helpdesk platform if needed.
Best regards,
DaumantasG