You have done pretty good job trying to solve the issue. So, right now the laptop does not power up and you get no noises from the PC, right? Since you have tested the power cables and you have replaced the motherboard, it seems that you may be facing a CPU problem. When you replaced the motherboard, did you change the thermal paste? Unfortunately, there may be a processor issue. For more troubleshooting options I advise you to contact the manufacturer’s support.
Hope this helps