My iphone was stolen. Please?

My phone was stolen and when I went to track it on Find my iPhone it was already offline. I put it on lock mode. But what happens if the thief resets it to factory settings? Do I get a notification when he/she connects to the Wi-Fi or am I not gonna get anything if they wipe all my data?
Thank you so much in advance!

Added (1). Also if I report stolen to AT&T would I still be able to use Found My iPhone with it?

Set new password. No hackers then.

Tell AT&T As soon as possible.

Call the service provider to track it.

Can I have more info on this?