I think the Things UNO can’t host the Sodaq Like firmware updater (flash space too short), so you’re right you need another board to update firmware and I don’t think you can do it connecting UNO Board to USB to a computer.
If you have any Arduino Zero, ESP8266 or ESP32 board somewhere you can try the forked firmware updater there that is working with other Arduino Zero, ESP8266 or ESP32 boards.
Of course, the board you will use for upgradding need to be 3.3V (not 5V) powered. And you need to connect 4 or 5 pins from your board (Zero or ESP)
3V3
GND
RN2483 TX
RN2483 RX
Option RN2483 reset
You can also power your upgrade board with 3.3V from things Uno board, no problem.
Let me know if it works