The app has been updated and the new Squad Assault mode is available to play.
The mode costs 20 stamina and it requires having 20 different units ready to battle through 5 maps to earn 2 orbs and a new seal called Squad Ace A 1.
It seems there are more assaults after finishing the first one which means more rewards down the line.
Chain Challenge seems to be a mode where you have to play combinations of the story maps to earn rewards, which go from feathers in normal and hard mode to orbs in lunatic mode. The stamina costs of the first two maps is 20 stamina with the last one costing 30 stamina regardless of the difficulty you're in.