Upgrade performance issues - Samsung Galaxy Note 4

Hi all,

I purchased a Samsung Galaxy Note 4 from a friend of mine in early May.  The phone was (is) locked to the AT&T network.  Everything worked fine - no performance issues whatsoever - until the Android update that was pushed through in late June.  Since that time I've been experiencing such severe performance issues that I've had to go back to regularly using my Galaxy Note 2.  These performance issues including crashing, freezing, and just plain unresponsiveness, and also occasionally difficulty turning back on after removing the battery.  In short, the phone is completely unusable.

I brought the phone in to my AT&T store in July, where they were able to perform a diagnostic and verify the phone was experiencing unexplained performance issues.  The store manager refused to take any responsibility on AT&T's behalf for the update crashing my phone, and insinuated this might be my fault in some way - despite the fact I had done everything he asked to demonstrate this was not the case (i.e., booting in safe mode, performing a factory reset, etc).  In sum, I've basically ruled out his assumption that I had just downloaded a bad app.

I was supposed to be contacted by someone in the regional office, but big surprise, that never happened.  I've recently dug out the phone and I see there's an update as of late August, but my phone doesn't see any update and is still running off the June 6.0.1 update.

Does anyone have any advice, either as to how to get the phone working, or as to the next steps I should take with AT&T? At this point, I feel they should take some responsibility in rendering my phone unusable due to their software update if they can't fix the problem.