From what you described it sounds like a big problem that can't be resolved. Since the cable and the sub board are ok the only big problem is the motherboard port. If the circuit between the motherboad and the main cable port doesn't register then there's a probability that the port is dead, you phone is bricked. Best thing you can do is to get a new motherboard. A main board to be exact. With the battery that is remaning transfer ASAP your data. But by the time your reading this I think it's scrapped for sure.