Finding a lost device without Find My app

The only way to find your iPhone is retrace your steps and look for it at restaurants and shops that you have visited to see if someone turned the phone in to the manager.

If the phone was stolen, I am sorry, but you will never recover the phone.

Have you tried calling it? Warning, if somebody answers do not provide account information. Do something like arranging to meet in a mutually safe location or have them drop the phone off at an office or someplace where you can pick it up. You don't know who found it.