Is your BIOS fully updated?
Well DDR4 2400 is actually only running at 1200mhz. Being DD (double data) you get 2 transfers per clock, hence 2400 rating. Check to make sure your RAM is in the right slots A_2 and B_2, refer to your manual, this is a common cause of memory not being useable.
try enabling all cores in the bios. sounds like they got turned off. i suppose asrock has the same ryzen setting as asus does.
oh and yes make sure you have cpuid 1.79 or newer or it wont show correctly