Since I installed Ubuntu 18.04, I can not switch to tty3-7 from the login screen. Until Ubuntu 17.10, this worked perfectly using Ctrl+Alt+F3, F4, .. both from login screen and from user sessions. This is a problem, especially if one is installing a fresh installation and is messing around with /etc/fstab.
I have read that the numbering of tty's changes, e.g. in How do I switch between console mode and GUI in 17.10? but I do now know why changing to terminal does not work from the login screen.
Is there a workaround for that?
Edit
When I try to switch, i.e. p.e. hit ctrl+alt+f4, nothing happens (not even a short freeze or so). If I log in and do the same thing, I see the terminal, can login and everything works fine. If I then lock the screen, I can still switch to the terminal.
My graphics card is a RX 480, the mainline drivers are (as far as I know) integrated in Kernel 4.15 and should therefore work. Furthermore, it works with Ubuntu 17.10 with Kernel 4.15 (mainline kernel), so I assume the graphics driver are not the problem.
Best Answer
This appears to caused by a bug in gdm3: Bug #1758512 "No virtual terminals (CTRL+ALT+F?) when no user logged in; erratic behaviour when user logged in."
I'd recommend going to that link and clicking the button to mark yourself as affected by the bug.