To be honest @jpsmithers I'd advise to return the phone for a refund due to the issues your having with it if the original owner isn't helping.
Logins like these have security protocols in order to protect the phone and any contents.
I wish you all the best with this.
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.