Cell error and reboots on own

These are fairly common issues with the 12 pro max and although they did replace the device in the past, they never actually fixed the problem.

This model, like previous ones, has a stacked motherboard. Meaning there's two boards stacked on top of each other and soldered around the edges. Those solder joints can become weak or break over time. Or in other cases the chip(s) themselves will have issues. This video will explain it and show how the repair is done. But this is not something you can DIY. https://www.youtube.com/watch?v=oAeOZeXR...

If you're in the states, you can mail your device into VCC and Jesse can repair it. I've personally sent him a handful of board repairs from my shop. He has excellent customer service and I highly recommend him.