iPhone not identifying headphones via lightning audiojack

I have just bought a Apple lightning to audiojack converter for my iPhone 7 and it is just not identifying any headphones I plug in and the phone continues to use the speaker.

• The volume is up
• The only speaker available is the iPhone
• The lightning port works as normal for charging / data transfer.
• I don’t have any other iPhones I can test the converter with.
• I have never used a lightning audiojack converter before.

I feel the problem is somewhere with the phone, I bought this iPhone damaged with the intention of fixing it (it sounded like just a bad screen repair) but I quickly found I couldn’t fix it. The proximity sensor and microphone have never worked for me.