You will need to perform a water damage repair on your phone, remove the logic board and clean it in an ultrasonic cleaner, then try a new battery and lighting connector. To be honest this should have been done before your phone completely shut down. Take it to a local repair shop, if you don’t have an extra battery, lighting connector, and ultrasonic cleaner.
How is the fan working, is it possble that the device is overheating. Blow out the inside and clean the fan and you made need to add new thermal paste to the CPU.