I have the same problem with my 6 plus. It keeps restarting after a few seconds and it does not show the Battery health. I have replaced its original battery with a brand new after market one about 1 year ago. The only way I can have my phone on for a long time is to connect it to a charger and put it in airplane mode with no wifi and not to get its screen unlocked. I am wondering if replacing the battery can solve the problem.