I have the exact same problem with my ASUS X550C, I have a splitter with 3 plastic splits (TRRS), which works fine on a friends similar notebook with the very same headset, and on my PC it doesn't recognise anything besides what's been there since I first started the PC.
I have already uninstalled and installed the latest version (which apparently was the one I had before, with AudioWizard instead of MaxxAudio (I'm not sure if it actually is called like that, but I do know the name is close) in the speaker tab) of Realtek HD Audio drivers from Realtek's website.
It's really frustrating to not being able to use my external mic, and have the people I speak with listening to every single key I hit when typing.
I've also look over the web, and haven't found any solution, nor cases that wouldn't be solved by purchasing the right splitter, which I bought thinking that it would be the solution, untill I find this topic which also leads to no fix.
Please let me know what can be causing this problem!
P.S.: When I plug the splitter with my headset pluged to it, the headset speakers work, but the mic doesn't. I'm sure it's not neither headset's nor splitter's problem because I tested it on a friends similar laptop, same brand, same look, just not sure of the model..