"Can you tell me how to add more frames from layers, to the frames that are already showing?"
You have option to duplicate frame at the bottom of Timeline (plus sign), New Frame from Timeline panel menu (hamburger icon) or add new layer in the Layers panel. If you have content on layers then duplicate frame and turn visibility of layers on/of (on for layers which you want to see).
Thank you Bojan for your response. The timeline panel was not showing me those options. But I opened it the next day, and everything was there. The option to create a timeline or slide view was missing. But it is back.