iPhone 6 plus seems Dead Weird problem

Hi friends, so I was using my iPhone one day and suddenly it got turned off and was not turning on, was not charging, no response at all. I thought it was dead I tried power button + home button to reset it, but nothing helped me but after approximately some hours I tried to power it on normally and it started working then again after few hours it stopped, I went to the local repair shop (Apple repair shops are not in my country) they said It seems software issue, they reinstalled the firmware/software with Itunes on my iPhone and boom it started working I paid them, then after almost one and a half day it again got dead completely, I went to another repair shop, he tried changing the battery but it was still not turning on still then he checked something inside the board and said It is dead he will try to fix it but there were holidays in my country for 4 days so he gave me back my phone and asked me to visit him after 4 days, I went home and kept trying pressing/holding the power button after every few hours but it was giving no response then after 2 days I tried charging it and it showed charging icon then turned on after few mins of charging.

what is happening please tell me I am just confused about what the problem is?? if it is dead why it is coming back after every 24-48 Hours and then gets dead after 24-48 hours or earlier.