as an english speaker, i would pronounce it "chat-oh", but i'm open to correction

At least here in colloquial "rolo" spanish people use to call "chato" (which would sound the same as "chatto") someone with a pug, snub nose

I don't know what the "official" pronunciation is, but I would say "Chat-o" is probably right.

Linguist here. It would likely be pronounced with a flap/tap, i.e., it would rhyme with shadow