Connect video input on the monitor to the output of the motherboard. Boot. Head into BIOS/UEFI setup by pressing 'Del', 'Esc', or some other key. Refer to your motherboard's manufacturer for the required key to press.
Next, find the option to change the 'primary video device' or however it's termed. Again, refer to your motherboard manufacturer's manual for this setting.
Change it to the external GPU.
Save and exit, then turn OFF the computer. Plug the video cable coming from the monitor into the GPU. Boot.
Thank you, I'll try this as soon as I can get a dvi adapter. I've heard that sometimes the hdmi port arrives dead on the card I have.