Almost supported: Xiaomi RA75 aka MiWifi Range Extender AC1200

For Developers
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I also figured out how to open the countdown for the stock firmware model RC04 for wait input keyboard. I want to share for those who need:

It's similar to this tutorial: Xiaomi Mi Router 4A Gigabit Edition (R4AG/R4A Gigabit) -- fully supported and flashable with OpenWRTInvasion but keyword
bootdelay => boot_wait

  1. Use CH341A for save Dump stock flash model RC04
  2. Use Hex editor: Find keyword: boot_wait. You will have 3 "boot_wait" locations, you need to change the last location boot_wait=off to boot_wait=on
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    image1280×720 39.2 KB

    Then save dump
  3. Use CH341A for Restore the modified dump in step 2 back to the Flash chip => then Uboot of RC04 will coutdown 5s for press keyboard into mode Flash TFTP