With an FTDI cable you can NOT program this microcontroller.
You can use it to read from the external Serial.
This is good to put the microcontroller into sleep and have some debug output running.
If you use this use the 3.3V and only connect RX->TX, TX->RX, GND->GND.
Our board is NOT Arduino UNO compatible but Arduino Zero.
What you need is an Atmel ICE to bypass the USB of the SAMD microcontroller.
And an TAG Connect cable to connect it to our SARA board.
But before we need to buy external tools. Reboot your pc.
Connect the SARA with the usb cable to your pc and double press the reset button.
If this causes the LED to fade, then the Arduino bootloader is still there.
You should get a new com port to program the microcontroller.
SARA rev3 has two usb ports, do you use the correct port?
The one next to the sim card slot?
Let me know if you get it back to life!