I have the same issue. My flex connector is broken at the cellular board and it is about 1/4 detached. There is not a place to solder it back and have not attempted to search for the flex cable. My ipad is not booting up and after about 3-4 minutes after the hello screen it reboots. Does anyone know if I fix the flex, then it will work fine again? I have DFU updated it a few times already without fixing the reboot issue. Also, it did not randomly reboot until after I removed the board to replace the headphone flex assembly.
Thanks