When running the Debian Installer via IPMI SoL, the GUI is powered by TMUX.
If you want to switch tabs to access the "2*shell" or the "3*shell" or "4*log" or get back to the "1- installer":

Press "Ctrl+A" then the number of the tab you desire.

PS: On a French keyboard (azerty) this implies doing 'Ctrl+A' and then 'Shift+&' or 'Shift+é' (évidemment), etc... Numpad also works.