All you need to do is visit more branches until you get approval. Insisting on speaking to a manager and arguing your case does actually make a difference. I got rejected by around 5 banks before finally getting approved, and all this was on a 30-day tourist visa. Do not accept "no" for an answer. Keep pushing, and you will eventually get approved. They might tell you a work permit is necessary, but it's complete nonsense, as they frequently approve tourists without work permits.

“Is Thailand worth it?”