All Z170 motherboards have had issues with overclocked RAM. Most seem to have been resolved with the most recent BIOS versions, however.
When I first booted my PC it was not able to post either. I was running 2666 sticks and had to buy a cheap stick of 2133 in order to update the BIOS and run the faster memory.
And how exactly did you do that? Just flashed to the newest version and you were good?
This issue occurs when factory overclocked RAM doesn't automatically scale down to 2133 and the motherboard doesn't yet support it's higher clock speeds so you can do a couple of things.
To guarantee it to work, you can install a 2133 stick and change the RAM speed multiplier from AUTO to 21.33 to force all RAM to run at 2133. This is probably the best option for you because, as some have mentioned above, RAM speeds have little difference in gaming performance. If you're feeling confident you can raise the multiplier from here and see how high you can go before you run into issues - I was able to get to 2400 before my board wouldn't POST with the shipped BIOS.
Option 2 is boot with the 2133 stick, enter q-flash and hope the latest version supports your RAM speeds. For me only the latest beta BIOS version supports my RAM when configured using XMP.
If any of the above stop your machine from POSTing you can always just swap any of your RAM sticks for the 2133 one and manually set the multiplier to 21.33 and start again. Just think of a stick of 2133 RAM as an essential Z170 system builder's tool rather than a piece of hardware - I keep one in my toolkit now! haha
Hope this helps
ducks