Addonprops, whilst pretty much obsoleted now (ever since OpenIV supports ytyp files itself now) should still work, though. So, you need to get that working first, as otherwise you'll simply keep adding these types of addons wrongly.
Another route you can go -- like I did for some stuff -- is to simply add to an existing DLC. For instance, I recently added an artwork to (my mods version of) mpapartment. So, I simply opened apa_int_mp_h_props.ytyp, and added the following entry (using an existing <Item> as template:
<bbMin x="-0.68351500" y="-0.05398700" z="-0.68227290"/>
<bbMax x="0.68351500" y="0.00042600" z="0.68227290"/>
<bsCentre x="0.00000000" y="-0.02678050" z="0.00000000"/>
'chair_artwall.ydr' being the prop I added to the folder (along with 'chair_artwall+hidr.ytd', in this case). Easy peasy.
Then I opened the _manifest file, and added the following segment:
That's it. You're done! The object is now properly registered with the game, and can be spawned accordingly.
IMPORTANT: As for spawning, objects inside addonprops, for instance, while internally already usable by the game (like when refered to in a ymap) will still show as 'Invalid' when you try and spawn them via an external tool. That is because Map Editor, or Menyoo, want to know the object exist too, so you need to add the prop name to either ObjectList.ini, or PropList.txt for Menyoo, respectively. So, when you say addonprops didn't work for you, make sure you actually added the object names to the aforementioned files too (if you plan to spawn them with those tools).