You cannot jump from firmware 11.00 down to 5.05 unless 5.05 was the very last firmware you had installed before the update. Why Do People Want to Downgrade?
Inside your PS4’s APU (Accelerated Processing Unit) are electronic fuses. When you install a new firmware, Sony’s code blows a specific fuse. If you try to install an older firmware, the system checks these fuses. If it sees that a fuse for a higher version has been blown, the installation fails immediately.
The PS4 motherboard stores two copies of firmware: the active one and the previous one. Because of this, you can generally only "downgrade" back to the that was installed on your specific console before the last update.