My iPhone 11 Pro Max will not turn on at all.

When I woke up my phone was completely off and would not turn on. I tried charging the phone but it would show no sign of life. I tried the volume up volume down power button solution and it still did not work. I also tried plugging my phone into my laptop to see if it will recognize the phone but it still did not. I took my phone to a shop and they said that it could be in diagnostic mode( I have no idea what that is) but they don’t really know. They also said that my phone is charging but at a very slow rate and they tried wireless charging and said it was doing a little better but still charging very slow. I just hope someone on here can give me a solution to fix my phone.