If you've you've managed to get your collectible stuck outside the showroom and can't move it back, here are a couple of tips.
Have you managed to get your collectible stuck outside of the showroom, or you need to reset its position?
How to reset your showroom / collectibles
Tap Settings on the Collections tab icon to launch the Manage Showrooms page.
On the Manage Showrooms page, click on the edit icon for that particular showroom
The Edit Showrooms page is launched which allows you to Rename/Delete or modify the collectibles added to this showroom.
You can remove the lost collectible from this page and can re-add it after entering the Showroom