You have to use the website, not the app! Then the mature stuff has a red “mature” label!
As Photo Otaku wrote you need to use a web-browser for tappytoon. The apps don't have any mature versions (because of US laws).
You of course have to create an account, but I guess you know that ^^.
Then when you search for "Dear Door" there are two entries. One of them is labelled with a red "Mature" remark. You might have to switch on "Mature contents" with a small button in the upper right corner of the homepage first.
Thank you thank you
So how you see the mature verison on tappytoon