My usb displaylink monitor works if I have already booted ubuntu with an hdmi monitor plugged into my GTX 1070. However, if I try booting with just my displaylink monitor plugged in, nothing appears on the screen.
It is possible that the driver for the adapter isn't loaded on boot. I would just unplug and plug in when needed, or add a cheap PCIe GPU for DisplayPort. I personally wouldn't bother figuring it out.
Nothing appears probably because the OS doesn't detect a monitor and the driver for the adapter won't be loaded. You'll need a monitor connected to the GPU for normal behaviour.