There could be some issues with the schedule - I would give it some days. You are right if you look up flights for 12 Nov on TG website - but if you try search Momondo or Skyscanner there should be 2 Thai Smiles flight later in the day. Fares are also quite high for this domestic flight now; most likely it will drop later. Did you check schedule of Vietjet?
Anyway, the last PG flight to CNX seems to be at 5:05PM
As a last resort you can take a taxi to DMK and take a flight from there. It is a fairly easy and quick transfer. There is even a (free?) bus between the 2 airports