I think those trying to help are confused about what you want to do. Do you want to send iMessages to this person, SMS messages, or MMS (picture) messages? If they have an iPhone you can always send iMessages, which can include pictures and text. iMessages can be sent over Wi-Fi or cellular data, so you only need one or the other (or both) on.
If you turn on "Send as SMS" the message will go as SMS/MMS ONLY if an iMessage is not delivered. Otherwise it will go as iMessage.
The only way to reliably send an SMS/MMS message to another iPhone is to log out of iMessage (not just turn it off) and stop using it. There is no way to force SMS to an iPhone if the other user has iMessage enabled and you do also.
If you want to send SMS you must have voice working on your phone; SMS uses the voice sidechannel. It never uses data.
If you want to send MMS you must have cellular data enabled; it cannot go as Wi-Fi. iMessage is the only one that can use either Wi-Fi or cellular data.