Samsung Galaxy S7 Edge System software not authorized by AT&T has been detected, also stuck in reboot loop
I purchased a Samsung S7 Edge about a year a half ago from AT&T when I was on my mom's phone plan. A couple months after getting the phone, I had to unlock it and switch it over to Straight Talk since she was switching to Verizon. It was fine up until a week ago. It froze and then restarted, and then got stuck in the reboot loop. I managed to get it into recovery mode and cleared the cache partition, but that didn't help. I took it to a tech store, (not an AT&T store) and they said since I cleared the cache, it sounds like a battery problem. Made sense since the battery was draining rather quickly over the past couple weeks. They replaced the battery two days later. I wasn't able to go pick the phone back up, so I also wasn't able to ask if they checked out the software or anything. When my friend brought the phone to me, it was on and the battery was at 37%. I thought it was okay. I tried to turn my brightness up, and it froze and restarted again and is stuck in reboot loop yet again. Every now and then, it will say "system software not authorized by AT&T has been found." It says to take it to an AT&T store for help. I cannot get it to turn off or anything. Now it also has an unlocked padlock symbol with "custom" when it reboots to the Samsung S7 screen. I can't get it into recovery mode anymore.
What do I do? Will a factory reset fix it if I do manage to get it into recovery mode? Do I take it to AT&T even though they aren't my service provider anymore? Can I take it back to the same tech store and ask them to check out the software? I have no idea what to do besides just buy a new phone.