1200 IS 2400.
It's DDR, double data rate, so 1200 is 2400. For DDR4 there CAN'T be any "1200mhz" memory, because there isn't anything supported at that speed for DDR4, and if there was, it would show up as 600mhz when reading the actual frequency.
Double data rate memory runs at a specific frequency, in this case it's 1200mhz, but it reads and writes on both the rising and falling edge of the clock frequency, which doubles the data rate to 2400mhz.
That still doesn't explain why you can't access the BIOS though.
I'd try resetting the BIOS as follows to see if you can gain access. If you can't, I'd RMA the motherboard. There is NO reason why you shouldn't be able to access the BIOS unless there is a problem with the BIOS ROM, or some other malfunction.
BIOS Hard Reset procedure
Power off the unit, switch the PSU off and unplug the PSU cord from either the wall or the power supply.
Remove the motherboard CMOS battery for five minutes. In some cases it may be necessary to remove the graphics card to access the CMOS battery.
During that five minutes, press the power button on the case, continuously, for 30 seconds. After the five minutes is up, reinstall the CMOS battery making sure to insert it with the correct side up just as it came out.
If you had to remove the graphics card you can now reinstall it, but remember to reconnect your power cables if there were any attached to it as well as your display cable.
Now, plug the power supply cable back in, switch the PSU back on and power up the system. It should display the POST screen and the options to enter CMOS/BIOS setup. Enter the bios setup program and reconfigure the boot settings for either the Windows boot manager or for legacy systems, the drive your OS is installed on if necessary.
Save settings and exit. If the system will POST and boot then you can move forward from there including going back into the bios and configuring any other custom settings you may need to configure such as Memory XMP, A-XMP or D.O.C.P profile settings, custom fan profile settings or other specific settings you may have previously had configured that were wiped out by resetting the CMOS.
In some cases it may be necessary when you go into the BIOS after a reset, to load the Optimal default or Default values and then save settings, to actually get the hardware tables to reset in the boot manager.
Do you even see the normal POST screen when the system powers on, prior to when it begins to boot Windows?