Reset factory settings keep photos and contacts

Make a backup, sync them to iCloud, or sync to a computer.

How to back up your iPhone, iPad, and iPod touch - Apple Support

Set up and use iCloud Photos - Apple Support

Change your iCloud settings - Apple Support

Transfer photos and videos from your iPhone, iPad, or iPod touch to your Mac or PC - Apple Support

You will need a back file of your iPhone that you can use to restore from after the iPhone has been erased and reset to factory settings. In other words, have a backup on either iCloud or on a computer.

Do you use iCloud to backup your iPhone including photos and contacts?

and/or

Do you backup your iPhone on a computer?

Axel F.