While activating 2FA on your STEX account, you get a 16 digit key that you must save. As shown below.
Scan barcode using app
To restore the 2FA in your authenticator app:
Open your Authenticator app add a new account, change the name to STEX and enter the 16 digit key that you have already saved while you activated 2FA on your account.
You may now continue using your Authenticator for logging into STEX.
IMPORTANT: Backup your recovery key!
Backup your 16 digit key/ barcode and put it in a safe place. If your phone gets lost, stolen, or erased, you will need this key to get back into your STEX account! Without this key, the only way to regain access will be filling a form to disable 2FA and can be a lengthy process.