You can buy a MIDI to USB cable at ebay for very cheap - less than £5 UK. They plug in anmd simply work on my system. So one end to the keyboard (turn down Audio) and the other end into your Computer via usb.

There are two kinds of synths avaiable - those that need a Sequencer to handle them these are called VSTs (there are other types too but they are less common). These programs require the sequencer to have the 'dll' extension file in their plugin folders. I beleive this can be done with Real Band but I never use it. Others may advise.

Then there are 'standalone' applications which run without a sequencer. There are less of these so you confine your choice by going this route.

The Good news is that there are thousands of free synths. Lots of them produce these 'spacey' sounds. KVR is a huge source - google.

If your bewildered by choice (and who would not be) 'Crystal' by Eric Persing (who also Designed Omnisphere one of the worlds best synths) is free, its pretty easy to use and sounds great. This is a VST if I recall.

Others are better to advise about how to load into Real Band

If thats too much try googling 'free standalone synth'

Win 11 64, Asus Rog Strix z390 mobo, 64 gig RAM, 8700k