I know a
Jera who pronounces it JAIR-uh.
This site lists it as an old rune, which means it was a glyph in the very very old proto-Germanic/Norse alphabet, the
Elder Futhark. It's far older than modern or even middle English. Google "rune jera" and you'll see what it looks like.
In the
Elder Futhark the J is indeed pronounced like a Y, but because that's such an ancient system I think it would be understandable to see the name pronounced