Add the MP3s to your iTunes library then sync that library with your iPhone.
oddly my windows 10 box stoppped recognising my iphone. It was using the version in the Microsoft store, and followed by about 4 mandatory patch cycles so at some time got messed up.Yeah it doesn't register with my PC, donlt think that USB (C?) port is working, I have the big USB's (A?) next to it that work?