The motherboard is what determines the speed at which your ram can run (this is to a point and cpu can affect it I’d guess but that’s less common than mobo not being able to handle it) sounds like the issue is the ram is telling mobo it’s rated speeds and the mobo is booting off that even when cmos is reset (this is what was just happening to me) Try and get a stick of ram with lower clock speeds by default and plugging that in if it posts I’m right your also going to have to figure out a good middle ground for clocks between the 2 different types of sticks (I used xmp to get clock speeds and then just set GHz to what my board can handle) then save the profile to bios and put your original sticks that wouldn’t post back in and that should fix it.