How to VNC to Ubuntu 19.04 without monitor while connected via SSH

I am trying to figure out how I can VNC to my Ubuntu 19.04 desktop which doesn't have a monitor or keyboard/mouse. I installed Ubuntu 19.04 on an external USB hard drive using my MacBook Air. I then booted Ubuntu on the MacBook Air and installed SSH. Then the above hard drive which has Ubuntu installed was connected to my headless desktop and Ubuntu boots up perfectly and I was able to connect to my Ubuntu desktop via SSH on my MacBook Air. I installed tightvncserver on Ubuntu via SSH and when I connect to Ubuntu via VNC using MacBook Air, I get a grey screen on VNC. Read many articles and tutorials using xserver-xorg-video-dummy and also using dummy plugs for DVI and none of these works. How can I have Ubuntu boot up normally as if it was connected to a monitor and connect to it using VNC? Seems like Ubuntu 19.04 is not booting up the X server window without a monitor connected. Please don't provide solutions that requires using thrid-party desktop environments when Ubuntu already has a perfectly working desktop environment which only functions when monitor is connected. I booted Ubuntu 19.04 on my MacBook Air and logged in with the normal GUI interface. I started tightvncserver and when I login to VNC I also get a grey screen.