Well, it's obvious that you won't be able to locate the phone using any kind of online Internet connection.....unless....the phone is connected to a Wi-Fi network with a good Internet connection.
We assume that Find My was enabled on the phone.....before.....it was lost.
If the phone is missing around home, the phone would likely be connected to the Wi-Fi unless the battery has run down or the phone was powered off.
If the phone was lost or stolen outside of the home, then there would be about a zero chance that the phone could connect to a Wi-Fi network on its own.
Use a browser on any device and enter www.icloud.com/find and enter the Apple ID and Password of the iPhone.
If the phone has a good Internet connection where it is located, it will appear on the Find My map.
I you see an "Offline" message, the phone is not connected to the Internet where it is located. About all that you can do is keep trying a few times each day to see if the status of the phone might have changed.
If "No Device Found" appears, Find My was probably not enabled on the phone, so any efforts to locate the phone will not be successful.