Can I use find my if the iPhone I can’t find says its offline?!!

If the iPhone is offline, there's nothing you can do.

1. If your iPhone, iPad, or iPod touch is lost or stolen - Apple ...
2. Find the serial number or IMEI on your iPhone, iPad, or iPod ... You cannot track using iMEI number. but you may need it so keep it handy

Did you enable Find My?

If you didn't turn on Find My [device] before your device was lost or stolen, it can't be used to locate your device. Change your Apple ID password. Change the passwords for other internet accounts on your device. This can include email accounts, Facebook, or Twitter etc...

Don'ts

If you remove the device from your account after you erase it, Activation Lock will be turned off. This allows another person to turn on and use your device.

Do's

• Mark As Lost.
• Erase your device.
• Apple Care+ :- file a claim for your lost or stolen iPhone. (If you had Find My enabled)