Chosen Solution
Recently my iPhone 7 Plus started not booting up after a housing replacement, first I thought it was the housing it self so I put it back in the original. Still had the issue, then I started in plugging something this maybe something might of gone bad after moving it to another housing but nothing seems to help until I unplugged the antenna near the cameras, the one resting on top of the flash. I noticed that it also got really hot when I tried to boot the phone up . Any reasons?
You need to look at that flex under magnification to see if there is any damage or tears. If it is damaged, then it will have to be replaced.
