Perhaps a year ago, I burned a Win10 ISO image to DVD using the Finder. Booted from it (using an internal optical drive), and installed Win10 to a spare HD. It was done under High Sierra at the time. Everything worked fine.
My Startup Disk does recognize the DVD as a boot option. But if that doesn't work for you, try booting while holding the "c" key. This will send your Mac to the optical drive.
btw - what Mac model (and year) do you have? Are you using a built-in optical drive, or an external over USB?