In brief ~
Fallen angels, were once holy, as all angels were created holy.
During the creation of the earth, Lucifer, became against God, and was cast to earth.
Drawn along with him went 1/3 of other angels. They lost access to their original estate, ie Heaven.
Nothing says "they" have been chained in hell.
Holy angels had access to move about back and forth; between their estate; Heaven, and the estate of mankind; Earth.
A few holy angels, came to Earth, and then defied God, mating with human women. They lost their access to their first estate: heaven and lost their ability to remain on mans estate; earth and were cast down to hell, where they are currently chain and kept in hell.
Taken