Traditional third party mods are not allowed in TruckersMP (local nor workshop), instead they will instantly be deactivated upon launching your profile in multiplayer. The closest thing you can do to modding through multiplayer is save editing. If this is what you are attempting to do, it may be that your modifications don't meet the guidelines, thus resulting in you being immediately kicked for having invalid accessories. Below are a few threads on the subject which you may find useful.