RAM Speed maximum, CPU or Motherboard based

So, I'm looking to upgrade the RAM in my Inspiron 3670, I'm currently running i5-9400 and motherboard is stock H4VK7 Motherboard. With the research I've done, the CPU supports DDR4 RAM with speeds up to 2666MHz, however when looking into motherboard specs, shows Motherboard only supports DDR4 RAM up to 2400 MHz Does this mean my PC will only use up to 2400mhz of the RAM speed even if CPU is capable of handling 2666mhz? if this is the case am I best off just getting the 2400mhz ddr4 ram?