To restore your Pillow Premium purchase, please follow these steps:
Ensure that you are logged in with the same iCloud/Apple ID that you used to purchase Pillow Premium in the past (all purchases are tied to their Apple IDs).
Open Pillow on your iPhone.
Tap on the "Settings" icon (cog icon) at the top right corner of the screen.
Scroll down and tap the "Restore purchase" button.