You presume correctly. Thanks. I do. It tries to send it as an iMessage and then comes back saying "Not Delivered." There is no option to send as text message, only to try again.I presume you mean that you can't reply. Are you getting any error messages?
Don't they have to be on the same wifi network?
Looks like no.I have originated and responded to SMS on my iPad Air numerous times without issue. It's one of my favorite new features! My 5S doesn't need to be on wifi for it to work, and I was surprised by that. SMS show up on both devices and it "just works"!