Will the auto-reply feature still function if my iPhone is turned off or out of range?

If you're referring to the Do Not Disturb setting for Auto-Reply, it would not work. I just tested it out by putting my iPhone in Airplane Mode.

It works just like a regular text message, needs a cellular signal to send back to someone. You can check with your carrier to see if they might have options.

Okay, thanks. I've never tried the DND while driving, so I guess it would send a response if you received a text, however without service, you are not going to receive a text message, so there is no way for DND to respond. You are looking for something that would work at the server end, which is the cellular carrier, and it seems you have already checked and they do not have anything.

Where are you planning on setting up this auto-reply feature? I'm not aware of such a setting.

The DND while driving feature will only work if the phone is turned on and has cellular service.