If you reboot, you loose your change...
Not when you save the changes to /www/pages/cgi-bin/minerAdvanced.cgi on the Z9 mini.
I believe what Django69 meant was that the changes are lost when the z9 mini is powered off, not reboot. The OS is running on non-persistent storage and any changes to the file system are lost when powered off. The config (ie. /config) on the other hand is kept on persistent storage and any changes here are retained.
When powered off, you lose your customized web page but your OC setting will still be set to what it was before the miner lost its power.
Then you and Django69 would be wrong.
I gave a solution that saves the changes to persistent storage (probably flash memory). It survives reboots, it survives power failures.
I have updated 10 z9 minis that all came with the "balance" code. Now they all have the freq drop down selector. I have rebooted them and I lost power on two by accident yesterday. Guess what? They still have the freq drop down.
I suggest you try this before giving out incorrect information.
FWIW With this new batch I start out the OC at 681 and drop back as needed until they run reliably. My slowest miner runs reliably at 662.