Audio jack splitter not working

I'm trying to get an external microphone and headset (separated) working on my 2015 15" Macbook Pro 2015. Unfortunately I run into a problem:

When I connect my apple earbuds (with integrated mic) in the headphone jack (just one) on my Macbook, both a headset and external microphone are being detected. Great, exactly how it should be!

However, when I connect a TRRS splitter only a headset is being detected, no external microphone.
I'm baffled why this doesn't work because it's exactly the same jack as the one used for Apple's earbuds.

The splitter I own is a Startech MUYHSMFFADW. I've seen video's using Startech splitters working on Macbooks so I have no idea why mine doesn't. I run the latest High Sierra OS version (10.13.3).

I've tried everything; plugging the mic into the splitter first and then into the Macbook. The other way around. I've tried mics using phantom power and one's without power.
Still, no external microphone is being detected.

Can someone help me out?

P.s. eventually I need the external mic to work in Bootcamp but first I'll try to see if I can get it to work in the OSX environment.