Well, technically [rɛindaːl], but “Rhine Dahl”, or even, “Rine Doll” is pretty close.
Although there’s a joke that it’s now pronounced “AES” since Rijndael was selected to be the standard’s algorithm. There’s even a wave file of it being pronounced as such, which serves both as the joke and the definitive guide to pronouncing Rijndael (he has to say “rhine dahl” in order to get the joke out). e
“Rhine Dahl” (Rijndael) is pronounced “A – E – S”.