Q: My back up only did the last month of photos?! How can I get my entire photo library backed up? I have plenty of iCloud storage.

Re you syncing photos using iCloud? If so, those photos aren't backed up.

Are you asking about a Photos ;library on a Mac or on an iPhone or iPad?

The iCloud backup of a mobile device does not include the items, that are already in iCloud (iCloud Photo Library, Scahed albums, My Photo Stream).  And iCloud does not back up your Mac at all. Use Time Machine to make a backup of the Photos Libary on a Mac.

When you do a backup under Settings>iCloud>Backup>Back Up Now, you are backing up the device itself, not your entire photo library. A backup of your device will only back up photos on your Camera Roll.

You need to turn on iCloud Photo Library so that those photos can upload to iCloud. You don't back them up, they upload via Wifi once you have iCloud Photo Library turned on:

iCloud Photo Library FAQ - Apple Support

iCloud Photo Library Help - Apple Support

Best of luck,

GB