I can’t imagine what it would be like trying to ride in the DR without a guide. And I can’t imagine trying to find a guide.
Despite the popularity of resorts around places like Puerto Plata, I don’t think people realise that the DR is actually a developing country, steeped in a history of poverty and pseudo-dictatorship (though not nearly as bad as Haiti right beside it). Unless things have changed radically over the last 15 years – and, of course, they could have – I can’t see how the country could have gone from little infrastructure to catering for relatively wealthy Americans and Europeans on expensive toys in a short span. Golf courses make sense because they are self-contained and can be run out of American resort hotels. But mountain biking? Unless some wealthy ex-pats have picked it up in Florida or New York and brought it back with them to set up a guiding outfit, I would be surprised if there was any opportunities there at all.
Probably better to explore the absolutely beautiful country on foot, or to find some locals to show you the real DR than to ride.
But I’ll be interested in learning how you get on.