Sorry? Confused now. You have the Cubase monitor turned off???? Surely this is exactly what the software monitoring button is for? Anyway - I still have 10.5 on my system, I'm on 12 Pro now - if you want to hear an input without using the direct button, the only other option is the L listen button above the fader in the mixer screen -this usually lets you listen to things you have put on the input channel, and does not send the signal to the selected channel fader, like the monitor button does, next to the record enable button.
Does the normal monitor button not do something? The listen workaround is sort of the same thing though? Maybe what makes the other normal solution not acce[table for you will be the same? It is the only other way to monitor the input that I know about?