Following this answer pointed out by V2Blast:
While mounted, you can choose whether to control it or let it be independent. If you choose to control it, it is limited by the normal rules to the Dash, Dodge and Disengage actions, but if you use it as an uncontrolled mount, it can do whatever.
While dismounted, the steed takes part in combat as a normal creatures would and:
When a monster takes its action, it can choose from the options in the Actions section of its stat block or use one of the actions available to all creatures, such as the Dash or Hide action.
And so it is free to use the Help action. Moreover, you are able to fully command it.