What I suggest is to take the guagua from RSJ to Nagua, Cariibe Tours from Nagua to Sanchez and guagua from Sanchez to LT.
Alternatively, maybe someone knows the schedule of the bus from LT to SD and back. Possibly you could connect with that bus in Nagua.
The bus from SD to LT does not go through Nagua - it uses the new road. There are three possibilities from Nagua.
Get gagua to Sanchez. This will get you to center of town. Take moto-concho to turn off to Las Terrenas.
Gagua from this turn off to Las Terrenas.
Gagua direct to Las Terrenas - I think about 2-3 a day.
Gagua to Samana and make sure you can get off at the turnoff to Las Terrenas after Sanchez. Thereafter pick-up to Las Terrenas.
To get to the gagua stop where the gaguas leave for Sanchez, Las Terrenas, Samana you may have to catch a moto-concho from the drop off point on Nagua.
Last gagua from turn off to Las Terrenas is about 5 p.m. so leave plenty of time. If you are late it is overnight an exciting moto-concho ride to Las Terrenas or a taxi.
From Nagua it is possible to negotiate a moto-concho direct to Las Terrenas via the new road - about 20-30 minutes. Be careful - get a recommendation from a shop owner etc.
Take plenty of small change to pay gaguas, etc. and be prepared to wait for a while with a cool beer!
yapask1