All modern PC monitors are RGB monitors ( every VGA, SVGA monitor )
But you cannot them directly with the Amiga, because the Amiga provides a 15KHz signal and a PC monitor can only synchronize to signals beginning with 30KHz ( and up to e.g. 85, 95, 110KHz,...depending on the monitor )
So that's the reason you need a scandoubler which doubles the Amiga's 15KHz to 30KHz, so it can be displayed by PC monitors.
If you don't have a scandoubler then you have the following possibilities:
1) Get an old 15KHz RGB monitor ( e.g. C= 1084, Philips CM-88xx models,.... )
2) Get an old multisync monitor. ( e.g. NEC 3D ). These monitors can sync from 15KHz up to about 45KHz and therefore you can connect the Amiga as well as the PC to them ( but you cannot use very high resolution on the PC because of the max. of about 45KHz. )
3) Hook the Amiga up to a TV that can understand RGB signals on it's SCART ( not all TVs can do that ! )
If you have an A-1200 or other Amiga that has composite-out then every TV with a SCART will do. You can also hook it to the TV using the modulator-out, but that will give you lower display quality than the composite-2-scart.
4) Again, if you have an Amiga with composite-out then you can also use the TV card method and have the Amiga display at the PC's monitor thru the TV card.
I think that's all methods I can think of right now.
So your 21" monitor will most probably be a modern PC monitor that starts with 30KHz sync and therefore needs a sacndoubler to work with an Amiga.