How can I find my lost iPhone 7 Plus?
There's no way you can locate your device without it connecting to the internet. You can go to this link and enable the lost mode. I am not sure if your phone was locked, if it was then there's no way it can have internet unless it is unlocked. If it was not locked and it gets internet access then the lost mode will be enabled and you can get the phones location. Also the phone will be locked after that. 

Once you enable Lost Mode, you will also get an option to enable 'Notify me when found', enable that option as well.

If your have not enabled Two-factor authentication for Apple ID and if you have any other apple device then I suggest you enable it by following instructions on this link. Because now there are high chances of someone trying to hack your apple id to use the phone.