HDMI

Make sure you follow these instructions: https://support.apple.com/en-us/HT202044

Also make sure the connector you have is authentic from apple (there are links to authentic dongles in the link I pasted above). Apple is known to release software updates to prevent third party connectors from working.

As an iPhone 7 does not have an HDMI port, you're going to need to provide more details for your question to make sense.

What are you trying to plug the HDMI cable into? What are you trying to accomplish by doing so?