: it's best for you to make a trip and find that "sweet spot". In my opinion, Rayong is not a city. It's even a bit inconvenient if you don't own a transport.
Top-up $ and select a new internet package on the app. The pre-paid number can be used for many years as long as you top-up and buy a new package whenever it expires.
DTV is done by yourself (if you fulfil all the criteria), not through agents. Submit an application at a Royal Thai embassy outside of Thailand. Beware of any agents promoting their service for DTV.