The gym must be registered properly and have all their paperwork for the DTV. I'm in pattaya and only a few offereing the service, but probably more than 50 gyms here for muay Thai. Find a gym in your city that knows the laws and has their licenses approved for the visa. It's not just a letter, the gym gives business license and other paperwork included for the process of getting the visa. FIGHTEVO360 in Pattaya ask for Susan, she is very knowledgeable and can answer your questions.
EVO360 muay Thai in Pattaya, owner name if Sharon and from the USA. She is very familiar and can explain about soft power dtv ... She has 100% acceptance so far. 20k baht for 6 months training gets you a 5 year visa.
Just go with the Muay Thai classes, EVO360 gym in pattaya talk with Sharon - 6 months 20k Thai baht. Heard some horror stories on cooking classes and prices.