No option to choose X11 on logging into Ubuntu 21.10
If you look on to the right bottom you will see a small icon, that can be clicked:
Select "Ubuntu on Xorg" and X11 is actived.
If you can't see the icon, go to your desktop, open a terminal:
sudo nano /etc/gdm3/custom.conf
Look for the line
#WaylandEnable=false
and remove the leading # sign.