When you enter Thailand, its counted as first report, which can only be done in person at immigration. Only if you extend, a dtv holder can do it online or mail. Based on what you asked I think its your first, so you need to go immigration. If Im wrong and you already did 1 and extended dtv, forget my comment :)
we were looking at driving to u-tapao and taking flights to udon thani, but there we would need other transports to reach the border... too complicated, but then a friend called and needed transport of stuff to khon kaen, so we didnt have to think the easiest way anymore :)
would need to use public transport, thats not for everyone... taxis from pattaya to airport and back around 3000 i think, so can count out flying if not going with a bus