Backing up my photos to iCloud

I have an iPhone 6s and have just realised it hasn’t been backing up my photos to iCloud. Guidance suggests that I go to Settings-ICloud and then switch on Photos in the ‘apps using iCloud’ section. But photos doesn’t show as an option in this section . Any suggestions about what I can do? I have checked and I still have plenty of storage in my iCloud and on my phone which was one potential reason it didn’t show.