Reinstall Big Sur with bootable disk
My wife upgraded to Monterey but some of her niche software is not compatible, so we followed tutorials on downgrading to Big Sur. We created the bootable disk, erased the startup disk, inserted the startup disk, start up pressing Option. We are given the option to Install Big Sur from the startup disk. We click that, it then asks for wifi. We select the wifi, enter the password and it that displays a spinning globe with text "Starting Internet Recovery". This goes for a while with the progress bar progressing, but then get the error 100f and it stops.
We have tried four times, the wifi is reliable and the bootable disk is still connected. It seems to get to the end of the bar and then the error appears.
Can anyone make any suggestions please?
Thanks