If this is your first 90-day after entering Thailand, the online reporting may not work. You will need to do it at your nearest immigration office with your TM30, TM47 and photocopy of your passport.
: 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.