Has the screen been replaced? This could be something like ''Long Screw Damage'' Try to put it in recovery mode and update it, it might give you an error code that gives you some clue. Also, what is the history? what’s happened for the device to be in the condition it is in?

I managed to sort this one. At first I swapped this U2710 and it made no difference, what did sort it was changing Tristar, I also changed Tigris before Tristar but that didn’t change anything, so it was a Tristar fault I believe. Successful case!

The SIM golden contacts look good? if not give them a swab with IPA and a Q-Tip, I’ve known this to cause SIM related problems, but if it is working completely fine in another phone then this might not solve it.

Got to the bottom of these and thought I’d share the knowledge. One phone had a short on "C3 image processor / Samsung S2MPB02 PMIC" (VDD_COMP_CORE_0P8) I didn’t think the caps looked bad so I removed the IC itself and the short went away, the device booted again with full image only camera not working, data is all fine though which was the main concern. The other phone however was not solvable. a full short to ground was found on VDD_0P8_CP by comparing board values. Two caps and one filter removed for this small line and short still present. Lower bigger PMIC removed and short still present. The only place left is the CPU, I had to call this one a day because replacing the CPU is out of my depths (even then it may be unique with the hardware encryption.

Although I doubted if the U2 was even the issue because power was getting inside the phone, replacing the Tristar IC did the trick. I usually like to refer to STS Telecom for this fault, they have a good video that I refer to if I need it: https://www.youtube.com/watch?v=XLFkvPA_... Problem fixed!