Google Photos Backup for iPad

Check your back up settings

1. On your iPad, open the Google Photos app .
   
   
2. At the top left, tap Menu   Settings .
   
3. Tap Back up & sync. If it's on, you should see these options:
4. If Back up & sync is on, you should see these options: 
   • Account: Make sure that the account listed is the same one you're viewing in the Google Photos app. To see which account you're viewing in the Google Photos app, open the Google Photos main menu and find your email address at the top.
   • Upload size: Tap this to change the storage size of your photos & videos.
   • Use cellular data to back up photos or videos": Choose whether you want to back up your photos or videos using your mobile network (rather than just WI-Fi). Charges from your service provider may apply.

Reference: Google Photos Help: My photos aren't backing up