I want you to realize and know one important thing and that is that there is NOTHING that you can do to increase the physical storage capacity of your iPhone. Your iPhone's storage capacity will forever remain the same as the day you bought it.
You can purchase ➔ iCloud Storage from Apple where you create and save your iPhone backups and sync (Not Store) your Messages, Photos, Contacts, Calendars, Notes and Reminders. Note that iCloud Storage is NOT an extension of storage capacity of your iPhone. Just wanted you to know some facts.
There is one thing that you can do make space on your iPhone when you save photos in iCloud. This is done by choosing to Optimize iPhone Storage under Photos in Settings.
Open Settings ➔ Photos ➔ Turn ON iCloud Photos ➔ Choose Optimize iPhone Storage
What does is that your iPhone saves the low resolution of your photos while maintaining the original resolution (Larger File) on iCloud. If you are just turning this feature ON then give it some time for the iPhone to connect to WiFi and perform this task overnight.
Axel F.