Put them both on an usb stick, plug it into the NAS' front port and restart it with the whole procedure.If you still get an output on the serial port you might be able to unbrick the NAS with re-flashing the uboot image.
SDK & Tools are also available on GitHub. Some pins are soldered to easily connect with a FT2232H board. Simple setup Connections from top to bottom = GND, VCC 3.3V, ADBUS0, ADBUS1.
In order to use it to protect keys, that’s a reasonable thing to do, but you know there’s still going to be the risk of attacks like Spectre, Meltdown, and Rowhammer,” says Will Drewry, principle software engineer at Google, referring to prominent examples of pernicious hardware-based attacks.