Using Garageband with audio splitter

Hello, so I saw that there are other posts about this topic but I can't seem to find an answer..

I used to have an IPad air 2 and used it to record my piano on garageband. I used an audio splitter cable in which I would plug the headphones and piano and everything worked perfectly. But I just can't make it work on the IPhone. The mic part is working so it is recording the piano but the monitor won't work, there's no audio at all. Not even while playing back.

I am using the exact same setup as I was on the IPad. I have no idea what I'm doing wrong..