How to unlock iPhone without data loss when in 'iPhone unavailable'?

Knowing the passcode is not sufficient if the iPhone status is Unavailable or Disabled.

Your data is not lost if you have a backup either on iCloud or iTunes.

If you see an iPhone or iPad unavailable message or a "Security Lockout" screen

If you forgot your passcode, you can use your Apple ID and password to erase and reset your device without a computer.

Learn what to do if you see "[Device] unavailable" or "Security Lockout" on your iPhone or iPad.)

iPhone is disabled.

Following this process is the only way out if your iPhone is disabled. The steps given in this link will help you reset the passcode -> If you forgot the passcode for your iPhone, iPad, or iPod touch, or your ...

Regarding your data…

1. If you have an iCloud or iTunes latest backup, then there is nothing to worry about. If not read more...
2. You have synced your data to iCloud, then what you will get back is what you have synced to iCloud and uploaded to iCloud Drive; the information that is synced and stored in iCloud are, Contacts, Calendars, Bookmarks, Mail, Notes, Voice Memos, shared photos, iCloud Photos*, Health data, call history, and files you store in iCloud Drive. (*-If you have turned on iCloud Photos, your content is automatically stored in iCloud.)
3. You have imported your Photos and videos to Mac/PC then they are safely residing there on Mac/PC
4. You have uploaded some of it to Cloud Drives using the Files app (iCloud Drive, Google Drive, OneDrive, DropBox, Flickr, box, AmazonDrive, Adobe Creative Cloud, etc). The data remains there.
5. If you have not backed up or do not have the latest backup, nor imported nor you have synced or uploaded your data to cloud drives prior to the situation that you are in, then I am sorry, all your data is lost.

I need to perform an iPhone rest, but I’m worried it will erase all my data.

The passcode reset will erase all of the data on the phone.

Is there any way to unlock my phone without erasing my data? 

Same answer. The passcode reset will erase all of the data on the phone.

All that you can do is use the most recent iCloud backup to restore data back to your phone during the new setup process. The phone will look just like it did on the date of the backup.

If you don't already have Apple's instructions to reset your iPhone and passcode.......

If you forgot your iPhone passcode or your iPhone is disabled - Apple Support

Sorry,. Your only option is to erase and restore the device. Anything that was not already backed up or synced to the cloud is gone.

There is no way past that.