But unfortunately, I hadn't realized that the mobile hotspot was sealed up in that update. So I looked it up, and saw that someone had recommended this:
So I went into TWRP and installed it. But that didn't work. When I booted back up EVERY app was crashing immediately except the stock Samsung apps (Messaging, Email, etc.). I tried to reverse it with the original framework-res.apk but when I did, the phone would get stuck at the AT&T logo (which was silent). I went back and forth a few times (when I applied that tethering mod, the phone would boot; when I tried to reverse it, it wouldn't), then did a factory reset in TWRP. After that, the phone booted, but the Setup Wizard would crash. So I did a Nandroid restore in TWRP and now the phone hangs at the AT&T logo no matter what I do.
My question is, what can I do now? I have a replacement device coming through warranty just in case, but it would be much better to just fix this one. However, I want to be certain and not HARD brick the phone...