I have a strix b450 F gaming MB, 3900x, G.skill 3600 ram. I cant get the ram to go over or even change from 2133mhz. If there is any other info some one would need to help me i can provide that for you.
Ram frequency @ 3600MHz is attained by overclocking. You can choose that speed in the memory frequency sub-menu in the Ai Tweaker menu but I doubt you'll have a stable system. Try testing out the system with choosing 2666MHz at first and see if it boots without any problems.
You need to boot into the bios and make the change theire I had the same problem as well. For some reason even though I had 3200 mhz RAM it was running @2133mhz. You may have a slightly different experience as I have an MSI version of the board, but you want to boot into the bios and find the XMP profile. I'm guessing it should be disabled, enabling should set the RAM to the correct timing. Alternatively, you can also leave it disabled and set the speeds yourself. Google is your friend for specific settings.