Quantum mechanical model of atom and the quantum numbers
So how come we are talking about fixed shells again and returning to
  what Bohr said?


Nope, we are not returning to the old quantum theory. We do not talk about orbits anymore, we rather have orbitals. Note that it is important to bear in mind that orbitals also aren't real, i.e. this description is still approximate, but it is a much better way of thinking about atoms (and molecules) than Bohr model. 

Now, to understand the concept of orbitals you have to learn the basics of quantum chemistry first, namely, the Hartree-Fock method. Until then just realize that:


We do use hydrogen-like one-electron wave functions (orbitals, or, spin-orbitals, to be more precise) to describe the state of each and every electron in any atom.
These hydrogen-like orbitals are described by a set of quantum numbers: the principal quantum number, the azimuthal quantum number, the magnetic quantum number, and the spin quantum number.
The set of orbitals belonging to a given principal quantum number $n$  constitutes what we call a shell in the orbital model.