iPhone App for External Microphone
No, there's no special app needed.

It sounds like you're using some kind of adapter/dongle for converting between the Lightning interface and a simple TRS. You do not mention what dongle or microphone, you're using.

It sounds likely that you are using incorrect cables. The TRS plug comes in two versions - the CTIA version used on iPhone (and most Android phones), and the OMTP version used on some Android phones and especially older equipment. If you have the incorrect verison, you'll get no voice recorded.