/ˈd͡ʒæŋɡˈmo.ʔo/
JANG-moh-oh
This is how it is pronounced in the Sun and Moon anime (see SM051 - Family Determination!).
The reason why the J is a voiced postalveolar affricate (the j in jump) and not a voiced palatal approximate (the j in hallelujah or fjord) is because Jangmo-o's name likely comes from the word jangle or jangling, in reference to its behavior of noisily smashing its metal scales together.
The latter part of its name is likely from Moʻo, the name of reptilian creatures in Hawaiian mythology; the - in its name is most likely a way to express a Hawaiian glottal stop (ʻokina) with a symbol that's easy to display.
