2133 Mhz is what cpu support on default. When I started my pc first time, my memery G Skill F4-3000C15-8GVRB were running at 2133mhz as well(i5 6500). I enable XMP profile and now they are running at 3000nhz.
bla, bla,.. if your RAM have XMP profile, activate it in your Bios, else "overclock" memory to 2400mhz.
EDIT: download CPU-Z and check under SPD tab, if your sticks have XMP.
I don't even have an XMP profile option :/ It basically lets me set the clock(auto/100/133) & the frequency(up to 2133). I doubt a new bios that had it would be available, this MB launched like a month ago. At this point I'm thinking all the sites saying this CPU supported 2400 were wrong but it's not worth replacing/sending anything back. I paid $3 more for the stick of 2400 than the 2133 & $20 more for the CPU I thought supported 2400.
Going forward on my next rig(w/same MB) would you suggest using the G4600 or just save $30 & stick with 2133. Again I know since it currently has no effect on mining it's probably silly to waste any more time caring about it, but if that's the case why do some suggest not going with the bottom of the barrel Celerons?