All meshes share the same pivot point.

That is, they all need to be position at exactly the same coordinates and with the same rotation, in order to line up properly.

For example, in the case of the vanilla Tel Mithrym interior cell each of the modular parts would need to be positioned with the following coordinates/rotation, in order to line up with the existing (large) mesh:

X Position: 177.3885
Y Position: 1469.5016
Z Position: 9.0029

X Rotation: 0
Y Rotation: 0
Z Rotation: 91.6732

If you are starting a new cell then it would make sense to zero everything out.