If you are waiting for Dungeon to arrive this month, there is no need to wait. Dungeons are closed. It will not be opened in March. This cooperative game mode will return in March with a new name. Here is what Kabam announced Officially:
So all the crystals available in the Dungeon store will not expire soon. Kabam will add a month to their expiring.
What will happen to the Dungeon Artifacts?
Dungeon Artifacts will not be lost. You will still be able to use them as Kabam mentioned. Kabam might change its name/icon to something else keeping the same amount of shards (just a guess, it may or may not happen). The point is: Your hard-earned artifacts will not be useless.