I am having this same issue with my S8+. My phone updated last night. When it restarts, it stalls at the AT&T logo. I have tried all suggestions on this site. I visited what was a big AT&T service center but told they no longer offer this service and recommended I call Samsung. Samsung sent me to an authorized repair shop for a free diagnostic check. They tried everything to include factory reset. I was referred back to Samsung. I have to ship my phone to them for repair. I will be without a phone for the next 14-20 days.
Really???? I'm being emailed repair estimates to fix something that wasn't my fault!!! I was told this would be repaired for free. They sent me 2 estimates today - one for $71 and an updated one for $216. I would never have sent my phone in if I was going to be charged. I've been on hold waiting for a rep for over 25 minutes.