I just bought a 1824 and I really beleived that it was "normal" that a simple monitoring were possible by the main output when there is no computer connected... I'm very sad that it's not the case. By Firepod did that 10 years ago !!
I manage to do what you are looking for, but it's not exactly "without computer"...
Start your computer once the morning,
wait for your Studio 1824 to be syncronize with the computer (I d'ont remember if the UC mixer has to be court-circuited in the options, try both...)
unplug the USB cable !
and then, without computer, the studio's monitoring will still work without the computer.
the on/off led will blink few secondes and then stay red, BUT THE MONITORING WITHOUT COMPUTER will perfectly work !
you can now switch off the computer.
It will work until you switch off the Studio product.
I hope it can help