How do I restore iPhone 14 Plus after corrupted update

My son cant have a conversation on the phone for more than 1 minute without his phone shutting down and restarting every conversation is cut off right at 1 minute it started after he corrupted his upload so we had to bring it into the store and basically he updated it on their MacBook and we went on our way it was about 2 weeks later my son is cut down to 1 minute conversations and shutdowns every few hours depending on his usage and he doesnt show up on my devices anymore in my find me it says cannot connect to server

[Re-Titled by Moderator]