Bus travel in the DR?

I’m going to a wedding in Punta Cana in July, and once the wedding is over I’ll be itching to leave the resort and see the actual country. I’ve found a bus that can take me from Punta Cana to Sanyo Domingo, which is great and I definitely want to see the city. I’d also really love to spend a night or two in Sabana de La Mar to see Los Haitises National Park. I can’t seem to find much information how to reach Sabana from SD. Is there a way to take a bus or busses to reach town? If not I could go to Samana which seems easier to reach, but I’d love to see Los Haitises if I can. Thank you!