What is the best way to get there if we fly into Cancun?
I'd recommend renting a rugged vehicle from one of the many internationally known car rental agencies located at the Cancun airport.
We made the trip South to Sol Carribe during the rainy season and rented a compact sport utility vehicle (SUV). The SUV did not have four wheel drive or even all wheel drive, but it did have a higher ground clearance than most other rental cars. You will need the higher ground clearance and sensible rough road driving skills (slow down & stick to the sides of any water hazard) and you should do fine.
Sol Caribe offers a pick up service, however, I strongly advice you rent a car, otherwise you will be stuck in the resort, as there is no public transportation within Sian Kan. Unless there are heavy rains, driving is slow but ok. The car will allow you to explore the jungle, the beaches, and the lovely Punta Allen village, where you can have a decent meal.
Get an answer from Sol Caribe staff and past guests.