I'm having the same issue. I have this mod, and a bunch of player models. If I create my own server, I can see and use all my player models in the selector. But if I join my friends server, who has this addon installed, i cannot select any of my custom player models. I can only use the default ones.

If we both have the same model installed, I can use it, but it shows as "error" in the selector.

If I have my own server and use my custom player model, my friend cannot see it, they only see a huge red blinking ERROR instead of my player model.