I purchased it in the past, but I want to install it again
You can download it as many times as you like as long as you use the same Apple ID.
You can install it from the App Store, but if it shows a Purchase button, please check the following.
Please try the steps below:
・Launch the App Store ⇒ tap the profile icon at the top right ⇒ Apps ⇒ My Apps ⇒ look for ComicGlass in the list, or search for it
⇒ If it’s not there, it’s not in your purchase history.
・You used a different Apple ID when purchasing (You can still install items purchased with another Apple ID, so is it possible you’re using the wrong ID?)