My and wife and I will be travelling from Rennes to Dinan on a Saturday this September. I find it difficult to find reliable info on the bus and train schedules. I prefer taking trains over buses, but in this case, I'm open to either one. Is anyone has better info, please reply. Thanks in advance.
The SNCF website shows multiple train connections between those places. Most of those are TER trains, so just buy the ticket at the station.
To complete WengenK's answer
https://www.sncf-connect.com/en-en/
September timetables are probably not yet available