Hi,
Please do try these troubleshooting steps that may help.
- First is to load in BIOS by pressing F10 while the laptop is booting up.
- In BIOS go to Boot Options, check if you'll get the external USB DVD drive and if it's listed set it as the first boot device.
- Save and Exit from BIOS then it should reboot automatically.
- If the external USB DVD drive is not listed in BIOS do try a different USB port then test it again.
- Next to try is to repeatedly press on ESC key while the laptop is booting up then see if you'll get the DVD drive listed.
- If all these will not work just create a bootable USB using Rufus and boot from a USB drive instead.