The ram list does not include my ram, but on the specs it says:
"Supports DDR4 2400 / 2133 non-ECC, un-buffered memory*"
"*7th Gen Intel® CPU supports DDR4 up to 2400; 6th Gen Intel® CPU supports DDR4 up to 2133."
And i have a 7th gen, which they claim supports up to 2400.
I'm either about to buy a different brand motherboard or buy a third set of RAM, which will probably let me down again.
are you sure the cpu is good?
are you sure the gpu's are good?
trouble shooting without a second mobo that can uses the cpu means you do not know if you have a good cpu.
basically
you have
bad risers
bad cpu
bad gpus
bad ram
bad mobo
bad hdd or ssd or usb stick
any of the above could be true.
so how to test a riser do it on a good setup if it works the riser is good.
how to test a cpu do it on a good setup if it works the cpu is good.
how to test a gpu do it on a good setup if it works the gpu is good.
the same is true for each and every part above.
if every part works they very likely the mobo is bad.
but there is not much you can do if you have just 1 mobo that can work the i5 7400 cpu.
since you know that does not work.
I know it sucks but you need multiple gear to test the issue out.