When placing modded items, they vanish from my inventory and the world

I have mods in, my minimap works, and my crafting guide. All the mods show up in creative. But when I try to place them down (in survival and creative) they show up for maybe a second and then vanish from my hotbar and where I placed them. ALSO, modded mobs won't spawn either. I'm very frustrated and would appreciate any help given :)) ty

They are installed for me? I wasn't aware you had to install them to the server. how do I do that?