I need to back up my photos before reseting unavailabe iPhone

if your iPhone is disabled.. it's too late to try and back up your photos.. In fact this is what a backup is for, you have to be proactive, not reactive.

If you were using iCloud Photos, your photos should still be in iCloud though (if you're using iCloud Photos, they won't be in an iPhone backup anyways)

Sorry, not possible. We will save the lecture about the importance of up to date backups.