Installation from USB options grayed-out

I have a Centos 7 server and I want to install AlmaLinux 8.5 on a separate SSD drive so I can do the configuration while still having the server running.
I have loaded AlmaLinux-8.5-x86_64-dvd.iso onto a 16GB usb stick using dd which I can mount as an iso9660 and it all looks good.
However when I try to do the installation I get to the customisation screen, but the installation source option remains grayed-out - see screen shot.

Is there a way around this blockage? Should I try the Troubleshoot option?
NB I have quite a few drives/lvs running on this server.

Mount as …”? You did get to this screen by booting from the USB, didn’t you?
When you do boot from USB, the default bootloader option is to check the image and start install, so one never has to explicitly mount the stick. Did you select that?

Of course I booted from the usb. The only reason I mounted it was to check that the data was properly loaded on the usb after it failed to get past the screen with the greyed out options each time on three separate boots.