How to fix 'iPhone Unavailable' without losing data?

There is no way to fix a disabled iPhone without losing data. Here are the instructions on what to do→If you forgot your iPhone passcode or your iPhone is disabled - Apple Support. If you have a recent backup you can restore that; if you don’t think about it for the future.

Your ONLY option is to erase and restore the device.

Any data that was not already backed up or synced to a cloud service is gone.

There is no way around that.