I am trying to find my iPhone but it is not listed under my devices on my iPad

Unfortunately, this is sounding like you did not set up Find My on the iPhone.....before.....it was lost. It is not possible to enable Find My on the phone, unless you have it in your hands to do so.

If this sounds like what has happened, I am sorry, but you will not be able to use Find My to try to locate the phone. All that you can do is retrace your steps, ask around and look for the phone the old fashioned way.