How do I get back erased photos from my iphone

If you have deleted them in the last 30 days:

• Open the Photos app.
• Tap Albums on the bottom menu. Tap on "Albums." ... 
• Scroll down until you reach the Utilities section and tap Recently Deleted. ... 
• Tap Select at the top-right of the screen. ... 
• Select every photo that you want to return to your iPhone.
• In the lower-right corner, tap Recover.

If you have deleted them earlier, there is no way to recover them without a backup

Check the Recently Deleted Album to see if the photos are there:

Photos > Albums > Recently Deleted

If the photos are not there, they are gone.

Recently Deleted album

or

Your last backup