The back of your a1200 without an a520 connected should have 4 phono size connectors.
modulator > for connecting aerial to tv
yellow composite > to monitor
red+white for sound
i doubt you will get a proper picture from the modulator but you should from the composite.
otherwise you need a 9pin>23pin monitor cable.
did you just get this amiga together with the monitor or have you had it working before?