I use my RPI4 headless mode on my PC every day. I also have it hooked up to my TV via HDMI cable in case I need it.
At this point, I am just going to step away and take a break for a few days before I get too frustrated. I have tried to use two different Kindle Fires, and my cell phone with no luck. Right now I have been trying to get it working on my Fire and I usually get the following message:
VNC Connection Failed!
Server did not offer supported security type.
Constant searches on Youtube and Google show a lot of people having no trouble at all so i know it works, just not for me YET. I am making a goof somewhere I just need to figure out where.