I need more storage on my iPhone7plus

See the following from What's the difference between device storage and iCloud storage? - Apple Support

When you buy your iPhone, iPad, or iPod touch, it comes with a set storage capacity. 

and

When your device is full, you can’t increase its storage. But you can free up space.

Also, you can buy more iCloud storage as described in the above support article but it can only help as described below:

• If you use iCloud Photos and turn on Optimize Storage, you can lower the amount of storage that your photos use on your device.