Train Ticket from Hua Hin to Hatyai Junction

I have been trying to secure 1st class train ticket from Hua Hin to Hatyai Junction on the 26th of December and have been checking daily for it's availability - unfortunately I kept getting 'waiting list'..my question is,does anyone knows when exactly the ticket window will be opened? ChatGPT kept telling me of the conflicting 180, 90 and 30 days to book so which is it?

Anyone familiar and able to answer?