iPhone 7 Plus restarting by itself
hello, I need help, I have this phone new I have it only less than a month, I have it from one gentleman so it is used and is not completely new, I have a battery capacity of about 82, I can not load it at the moment I do not know why, but as I say the phone I really have less than a month, it worked perfectly but suddenly out of nowhere it started to restart by itself, it's already 3 days and for today (18.6) it restarted about 8 times and I don't know what to do anymore, thank you in advance for the answer...
[Re-Titled by Moderator]
iPhone 7 Plus, iOS 15