8 gb ddr4

Hello! I have a problem with my RAM and I decided to upgrade it, but I don't know what to choose. Now I have 1x 8 GB DDR4 2133 MHz Crucial and 1x 8 GB DDR4 2400 MHz J&A, but the problem is that my PC won't boot in dual channel(I have motherboard MSI Z370-A Pro and CPU i7-8700). What is the best...