React Native bluetooth device authentication

I'm trying to communicate with a bluetooth LE device, but have been told I need to "authenticate" before being able to read / write data. The hardware dev has told me that the device sends a key to the recipient, and I need to reply with 12000000000000000000000000. He has tested this successfully with the NRF Connect desktop app (but I need to replicate this in react native).

I've tried sending 12000000000000000000000000 (converted to base64) to the device's notify characteristic as soon as I connect to it using the code below:

const Buffer = require("buffer").Buffer;
const loginString = "12000000000000000000000000";
const hexToBase64 = Buffer.from(loginString).toString("base64");

characteristics[0].writeWithResponse(hexToBase64).then(()=>...)

However, I keep getting "GATT exception from MAC address C7:7A:16:6B:1F:56, with type BleGattOperation{description='CHARACTERISTIC_WRITE'}" even though the code executes properly (no catch error).

I've looked through the react-native-ble-plx docs and still haven't found a solution to my problem, any help would be apreciated!

from React Native bluetooth device authentication