Hello since i am still newbie i cannot send private message to you. Can you please send me in pm link of that firmware (Filename: Antminer-Z9-Mini-201805262047-500M.tar.gz) , there is none at bitmain page anymore. Thanks in advance!
Here you go - just in case it's the firmware that's locking down overclocking and not the 'silicon lottery'.
https://drive.google.com/open?id=1y3poGmeOpeZuAy4rvfT4uZ1Z7tozjTLxCheers!
EDIT: Had inserted the wrong link by mistake initially. Have updated it. Should be working now.