Try and see if the settings application shows the onscreen keyboard as enabled or not. Should be in All settings- Virtual keyboard. If it was working on the lock screen it might just need some setting to change. Are you on X11 or Wayland? That might also be a factor, if it stops once it switches display servers.

Which distro are you on right now? It should have it, at least there is one on mine. Think it’s called maliit-keyboard.