Using a headset with microphone on Macbook Pro

I have a 2019 Macbook Pro (15") running mcOS Monterey version 12.3.1

I purchased a gaming headset (Red5 NOVA headphones with built-in microphone) and since the macbook only has one 3.5mm port I was advised to buy an audio input/output splitter which I have done (SYNCWIRE 3.5mm audio splitter male-to-2x female for connecting mic + earphones). All devices checked and advertised as being compatible with Mac/Macbook.

In the Sound settings the output pane recognises "External headphones" but in the input pane only the mac's built-in mic is an option.

What else do I need to do so I can use the headset & mic together? It's for taking and making calls in various programs like Jabber, MS Teams, Skype, Zoom, Facetime.

I can use the headphones and the Mac's built-in mic together, but that's not the point, I don't want the built-in mic on the headset to be non-fuctional. I have tested it on a PC and it does in fact work, but it's essential for me to use on the Macbook Pro due to work.