@Maw19: An external backup would need to have been created in order for you to recover your photos. If you synced your photos to OneDrive via the Gallery app, then please open the app > Tap the 3 lines in the bottom right > Settings > Sync with OneDrive, and follow the on-screen instructions. Similarly, if you have registered the same Google account to your new device via Settings > Accounts and backup > Manage accounts > Add account > Google, then please swipe up from your Home screen and head to the Google folder > Photos, where you should find an archive of your images (Wi-Fi or mobile data connection required).
It might not be possible to repopulate your Gallery with the backed up images, however the steps above should hopefully allow you access to your photos, so you can be assured that they are not permanently lost.
If you haven't done so already, you may also wish to take a look at Find My Mobile, as this feature allows you to remotely manage lost and stolen devices, including the option to backup your data to Samsung Cloud.