Why custom stuff falling apart

I’ve made a simple script that allows players to pick up stuff etc…
And i wanted to make a box that i actually did (no materials/textures yet but i have the model) and to make the script that allows players to pick up stuff work i need to turn on simulate physics.But whenever i start the game my “mesh (box)” falls apart (- YouTube) is there any way to make this work ? My english is very bad so if you didnt understand i included a video! Is there any way i can like compile them into one mesh so i can simulate physics and it wont fall apart.

Make the box into a single static mesh. Does it need to be made out of separate elements?