How can I find my iPhone location when it’s offline?

You can’t generally. If you enabled the Find My Network when setting it up then it may be possible to locate it with the crowd-sourced Find My network, if someone with a recent model iPhone comes near it. You would check the same way you would if it was online: https://icloud.com/find

"Offline" can be the result of several different things or a combination of them.

• The phone might be powered off
• The battery might have run down
• The phone may not have an Internet connection where it is presently located

All that you can do is keep trying every 3-4 hours to see if the status might have changed.