This is just a longer and more fully documented version of what you were advised in an earlier post:
'If you're missing photos or videos in the Photos app' on a mobile device, click here --> https://support.apple.com/HT205912
- Check your Recently Deleted folder
- Make sure that iCloud Photos is turned on
- Sign in with the same Apple ID
- Unhide your photos
If you were not using iCloud Photos then you will have to see if they are part of a backup you made:
Read/watch these suport items -->
Click on this link to discover how to 'Restore your iPhone, iPad, or iPod touch from a backup' --> https://support.apple.com/HT204184
or watch this video if you're using a Mac to do this:
https://www.youtube.com/watch?v=Jbp3gGTQygg
Related information:
Use iCloud to transfer data from your previous iOS device to your new iPhone, iPad, or iPod touch --> https://support.apple.com/HT210217
Some things are not stored in a backup. Click on this link and read "About missing information after you restore your iOS device with iCloud Backup" -->
https://support.apple.com/HT203768