my iPhone keeps restarting

Hi,

My little brother got a preowned iphone 7 a month ago with 90% battery capacity and yesterday it started a loop where it will restart every 5 minutes. I've already reset all settings and configurations from the settings app, I've taken out the sim card several times and even reset and updated the phone throught iTunes using the DFU mode.

Also, when i try to check the battery health, nothing will come up, it keeps loading and loading until it restarts again.

At this point i don't know what to try and i'm starting to think it a hardware problem more than a software one. Plesase help.