MOTU Express XT on Mac - No Signal at Sound Modules

Hi!

I'm trying to help out an older friend with his setup and I've hit a bit of a roadblock. I'm not hugely familiar with MIDI so I thought I would post here.

He has:

Mac Mini - running Ventura 13.6
Audio interface - Behringer FCA1616
2 x MOTU Express XT Midi interfaces (these are connected via USB to the Mac)
Midi Controller - Arturia KeyLab Essential mk3 49
A bunch of old MIDI sound modules, e.g. Roland R8-M; Proteus 2000; Alesis DM-5 among others...

The Behringer unit is currently working so we have that up and running. I've been working today trying to bring the MOTU units and the MIDI units into action.

After following some different guides online, I have managed to get the Mac to recognise the MOTU units - the MOTU software shows them, and the MIDI mapping software on the Mac shows all of the units connected and powered on.

I'm not really familiar with Reaper, but I have managed to select the Arturia as the MIDI input so that I can see the key inputs. When I select a VST instrument as the FX I can play through the instrument.

My question is: I'm not quite sure how I go about setting it up so that the MIDI signals can play through the MOTU units and use the MIDI sound modules.

When I have selected the MOTU unit and channel, e.g. 1, on the MIDI output on the track, I can see a signal reaching the MOTU unit - BUT, it shows on the 'MIDI Out' lights on the MOTU unit.

Something is making me think that the MOTU unit is wired up wrong, but I'm not sure. Or maybe that I'm not setting up the track properly.

My other thought is - should the MOTU units be hooked up to the Behringer Audio Interface? They are only connected to the Mac Mini via USB, should anything be routed via the MIDI ins/outs on that unit?

Sorry for long message, tried to give as much info as possible for context.