Rooting Problem

I purchased an AT&T GSM phone unlocked and I use Straightalk as my wireless carrier. I have rooted before and used the newest version of Odin. I downloaded the the proper file for root "G928A_Kernel_Root_5.1.1_Samfirm.net.tar.md5." My phone model Is SAMSUNG-SM-G928A. It's the one everyone else used. Odin flashed it fine. It passed and the phone restarted. Problem is the phone is not rooted, but I have the problems that come with the initial root. Wifi passwords don't save and the annoying message "Unauthorized actions have been detected". Since the phone is not rooted I can't fix these problems because I don't have permissions. My next step was just to flash the stock firmware but I can't find it anywhere. Can anyone help me either tell me how to truly root the phone (I re-flashed the file 3 times) or tell where I can download the stock Firmware? I called AT&T and they told me to call Best Buy and Best Buy can't help me.
Thanks

Samsung s6 edge+
Samsung-SM-G928A
Android Version 5.1.1

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