So we know how you proceeded with the new installation and current hardware, please answer.
Did you do a bare Bones test of the replacement Mb?
Are you using the Corsair H100i V2, or an air cooler?
The COM port on the MB, Page 4 #20 of the user manual, is a header to add a Serial Port extension bracket to the back of the case, you will not need to use it.
The case PWR/Reset/HDD/LED plugs into the F-Panel header, page 4 #13. then read from page 29 #4. The plugs usually come as small 2 pin ones, they will be marked, pwr/rst/hdd/led. They should have a + or arrow indicating the positive(P power) wire, Match that wire with the P pin as instructed on the diagram.
The COM cable you are referring too, sounds like it is the front USB 2.0. Page 4 #16 and page 22 #5. If you look at the diagram, the blank spot is at the bottom right (as against the com port, blank spot top right.