How can I find my lost device if iphone is continuously going offline

If Find My iPhone is offline or not working, read this Apple Support article to get help; e.g., when you don't see your device listed, or you see an unexpected device or an alert that says your device is offline. --> If Find My is offline or not working - Apple Support

This support article has a list of things to do if your iPhone, iPad, or iPod touch is lost or stolen. --> If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

- The Find My service described in the article is the only recovery feature Apple offers. It must be set up before the device is lost.

The iPhone will display as "Offline" if the battery has run down or the phone does not have a good Internet connection where it is currently located.

There is unfortunately nothing that you can do about this except keep checking a few times each day to see if the status of the phone has changed.

Also, you may not be aware that thieves are very fond of placing a stolen iPhone in a special type of bag that will not allow the phone to send or receive a signal.

It's too early to throw in the towel, but realistically there is almost a zero chance that you will ever see the phone again.