How to unlock iPhone stating device is unavailable without losing data?

I understand that your iPhone is in the state of "Unavailable". You will have to reset the passcode to your iPhone by following the instructions in this guide published by Apple 👉 Reset the Passcode of your iPhone (Without a Computer).  Note: All the data on your iPhone will be erased. There is no way around this.

Hopefully, you have a backup created on 👉 iCloud and/or on a computer. If you do then, you can 👉 Restore your iPhone from the backup file once your passcode has been reset and the iPhone has been returned to factory settings by the procedure in the link above.

Axel F.

You don't. Once your phone became unavailable, your data was gone.

You will have to reset you phone and the restore from a backup if you made one.

If you forgot the passcode on your iPhone, or your iPhone is disabled or unavailable -

Click on the link for instructions on how to reset your passcode without a computer if you have iOS 15.2 or higher.

If you see an iPhone or iPad unavailable message or Security Lockout screen - Apple Support

Click on the link for instructions on how to reset your passcode using a computer.

If you forgot your iPhone passcode - Apple Support