How can I track my phone even if it’s off

If Find My isn’t showing a location, it’s not trackable.

If the iPhone is off or out of battery and/or if the Find My network option was disabled, or the iPhone out of communications, it cannot be tracked.

Find My info: iCloud+ - Find My - Apple

If you don’t have another Apple device to use with Find My app, use iCloud.com/find

And for completeness, there is no means available to users to track via serial number or IMEI. Find My is the available means.

You cannot.

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support