I can access download mode and recovery mode in my SM-G870a ATT but it won't boot

A

AC Question

I've been working on this phone for the past 10 hours and I'm hitting a dead end, but I need some ideas. I can't find any firmware for 5.1.1 that would successfully install through the recovery mode. The one on Sammobile is 4.4, the bootloader is locked so I can't downgrade my Android version

The phone originally got messed up when I followed this guide.

1. Take the NG3 OTA update.
2. Download and install Towelroot-v3.apk(this version does not require internet access).
3. Place your phone into "Download Mode" (press and hold the Home, Volume Down, and Power keys until the phone powers on and displays warning screen, then press Volume Up key) and flash the G870A_NE4_Stock_Kernel using Odin's PDA slot(AP in Odin v3.09+).
4. Open the Towelroot app from your app drawer and press "Make it Rain" to root your device.
5. Power off then boot into "Download Mode" again and flash the G870A_NG3_Stock_Kernel also in Odin's PDA slot(AP in Odin v3.09+).
6. Download SuperSu to manage root access. Done!

At the time I did not realize that the update made rooting my phone not possible right now.

Please help me I feel so dumb for making such a huge oversight to even check if my current firmware was rootable. I just assumed it was. I promise to follow instruction very carefully and am willing to try anything to get this working again.

Thanks