Hi, if you have addon packs installed, and the server does (which at this moment, I believe you cannot add addonpacks to Aternos, but I could and most probably be wrong) not then you cannot inject it into you. If this is not the case, then restarting your server and/or relogging from the server could fix it. If not, I have no clue, I'm sure someone will find the solution out there.
Edit: It always glitches, if you don't have addon packs installed, even on singleplayer. I have to close & re-open the world, AKA restarting the server on multiplayer.
Hope this helps!
Kind Regards,
Frost