How do I set custom model/texture data of equipped armor?

I’ve made an elytra that has custom model data, and the model shows up just fine in the inventory, there is no issue with the new textures as long as you don’t equip the armor, but as soon as I equip the armor, it goes back to looking like the vanilla elytra (From F5, it still looks right in the inventory). What I’m wondering is how to set the custom model data for equipped armor as well, so it isn’t just changed when it is in the inventory. I’ve seen a datapack that does that, but after looking through it, I wasn’t able to figure out what they had done to get it to work.

Any help is much appreciated, thanks in advance!