Unless you know if the logic board is good, nothing we can do at this time.
Do you have any friend who is willing to lend you a phone that you can swap out the logic board to see if yours is still good.
There are so many possibility wrong with your phone.
1. Power button doesn't work. Therefore, you can't turn it on or put the device into DFU.
2. Your Iphone charging dock might be defective.
3. USB cable is also another possibility (this is lightly an issue.