Just sharing a quick fix for an issue you may encounter when working in secure environments.
I encountered a Hyper-V VM in the lab today that would not boot from an Ubuntu ISO image. No matter what I did, the VM went straight to attempting PXE boot (even with network boot at bottom of boot order list).
Then, I noticed in the VM Settings, the Enable Secure Boot option was checked. This feature (available only on gen 2, UEFI VMs) prevents unauthorized code from running at boot time…which includes random ISOs mounted to the VM.
Uncheck the ‘Enable Secure Boot’ option, and the VM will boot from the ISO as expected.
Read more about Secure Boot at https://blogs.technet.microsoft.com/dubaisec/2016/03/14/diving-into-secure-boot/