1st. What event happened that caused this issue to start?
As to what it sounds like may have caused this. Possibly replacing the front screen and replacing the original home button, i ve read on apple forums that a fingerprint model of iphone home button being replaced by a aftermarket home button.
A much more common issue like this happens 6-12 months after a cell phone was jail broken. The code written by the pirate coders has bugs, etc in it, that slowly over time corrupts the apple software so badly that after 6 months a problem like this arises. If thats the cause, there is no cure, sell it on ebay and let a expert in microsoldring buy it. They can just start swapping soldered parts on the board until they isoliate the corrupt microprocssor on the board.
Was the phone exposed to a liquid spill sometime in the past 4 months? Its about that amount of time it takes a tiny area that was touched by water takes befoe the corrotion
One other you can check is to use a 10x-20x glass and a flashlight to look closely inside the simcard tray slot. Look for a hair, dust, also exaime the sim card and see if one area of the copper has a unusal deeply wore area verses the others. Also inspect all the other orafaces for any debris that might cause a short.
Lastly, the simcard reader might be bad
Only connecting a MICROSOFT PC via USB to a iphone and when itunes opens on its on, will activate the APPLE FIRMWARE REPAIR diagenostic tools that you need ran to fix any software issue repair.
First i would charge the phone to 100%, then wipe the iphone and restore it as a new iphone. Before ever connecting it to itunes.
iPhone 6 plus boot loop after screen repair
refer, to the post 9 down below yours, called. iphone stuck in loop