Can a creature with a natural walking speed of 0 ft. get up from being prone?

In a one-shot I recently played the players were fighting a homebrewed Weeping Angel (stats). It has a walking speed of 0 ft. and can teleport 80 ft. on its turn under some conditions. One of the players knocked the Weeping Angel, which is essentially a statue, over by using the shove action. On its turn, the Weeping Angel was prone with a speed of 0 ft. It then used its Impossible movement to teleport away and appear upright again.

Now there are three options on how to rule this situation:

  1. The Weeping Angel uses half of 0 ft., so still 0 ft., to get up from being prone and proceeds as normal
  2. The Weeping Angel lifts the prone condition simply by teleporting
  3. The Weeping Angel is prone, and can’t get up anymore

According to the rules, which of this rulings (if any) would be correct?