X11 VNC Server

Install x11vnc and xserver dummy (in case you wanna use Desktop without monitor)

sudo apt install -y x11vnc xserver-xorg-video-dummy

sudo nano /etc/X11/xorg.conf

Section "Device"
   Identifier "Configured Video Device"
   Driver     "dummy"
   VideoRam   40000
EndSection
Section "Monitor"
   Identifier "Configured Monitor"
   HorizSync 22-83
   VertRefresh 50-70
   Modeline     "1920x1200_60.00" 193.16 1920 2048 2256 2592 1200 1201 1204 1242 -HSync +Vsync
EndSection
Section "Screen"
   Identifier "DefaultScreen"
   Monitor    "Configured Monitor"
   Device     "Configured Video Device"
   DefaultDepth 24
   SubSection "Display"
       Depth 24
       Modes "1920x1200"
   EndSubSection
EndSection

Systemd

sudo nano /etc/systemd/system/x11vnc.service

[Unit]
Description=System service for x11vnc connection
After=display-manager.service network.target syslog.target

[Service]
Type=simple
Restart=always
User=ubuntu
ExecStart=sudo /usr/bin/x11vnc -env FD_XDM=1 -auth guess -shared -forever -autoport 5900
ExecStop=sudo /usr/bin/killall x11vnc

[Install]
WantedBy=multi-user.target

sudo systemctl enable x11vnc.service
sudo systemctl start x11vnc.service