This probably won't work.
In addition to the answer by Thomas Markov (this item is homebrew and needs a DM ruling), there's a good chance it won't work.
The ring is based on the Alter Self spell's "Change Appearance" feature:
You transform your appearance. You decide what you look like, including your height, weight, facial features, sound of your voice, hair length, coloration, and distinguishing characteristics, if any. You can make yourself appear as a member of another race, though none of your statistics change. You also can't appear as a creature of a different size than you, and your basic shape stays the same; if you're bipedal, you can't use this spell to become quadrupedal, for instance. At any time for the duration of the spell, you can use your action to change your appearance in this way again.
The reason it's likely not going to work is the part I highlighted above: you can't change your basic shape. That means that your Spined Devil will not be able to mask their tail or wings, and will never look passably humanoid.
However, regarding the abilities, the Devil will retain all of them since the feature also says "none of your statistics change".