First thanks for the format used when posting!

I am going to admit a lot of first time/elementary mistakes when getting back into the M5Stack environment with my M5Dials (it has been several years). I will admit that it is strange that the DEMO firmware will work but not the actual firmware?

do you have any other controllers to verify your configuration?

is the USB cable OK is the USB Port working correctly

are you downloading the correct firmware for UIFlow2? (I didn't LOL, I used the firmware for UIFlow1)

are you using the correct USB Drive for your Operating System (I had had to try different versions until I found one that worked).

try downloading a different version of the firmware for now

It sounds like you discovered what button you had to push to get the AtomS3R into download mode. I also made that mistake.

Could you post back once you discover what is the problem?

Thanks-Brian