i know you have said you have done a reset but this is all i could find out.
To perform a hard reset, hold the power button and Home button (the circular button below the screen) at the same time until your
iPhone screen goes blank and the Apple logo reappears. ... People were amazed when they'd come into the Apple Store and I'd quickly fix their dead
iPhone with a hard reset.22 Sep 2017
Why does my iPhone keep restarting itself?If iPhone iPad Keeps Restarting Randomly. You can restart or force-restart youriPhone iPad to clean all processes, which may help you fix "keeps restarting" issue. To force restart one device, just press and hold both of the "Sleep" and "Home" buttons at the same time for 10 seconds until Apple logo appears on screen.
What do you do when an Iphone keeps turning on and off?To do so go to settings ? Cellular/Mobile ? Turn On/Off Mobile Data. RebootiPhone: To reboot your iPhone, hold the power button and home button simultaneously for a few seconds. The Apple logo will appear on the screen and your iPhone is rebooted.24 Mar 2014SOLVED: My iPhone keeps rebooting/shutting down by itself? - iFixithttps://www.ifixit.com/Answers/.../My+iPhone+keeps+rebooting-shutting+down+by+itse...Search for: What do you do when an Iphone keeps turning on and off?
or go to this link.
iphone 6 keeps rebooting Google Search