Ubuntu 19.04 doesn't detect my HDMI display

The external connector DP-0 is hardwired to the nvidia gpu so is only available if the nvidia gpu is actively rendering. No way around it.
nvidia-smi sees no device because on intel profile the nvidia driver is blacklisted, you would have to manually load it.
sudo modprobe nvidia nvidia-modeset nvidia-drm nvidia-uvm
or just enable nvidia-persistenced
sudo systemctl enable nvidia-persistenced