I phone 4S IMessage cant force wifi

I live rural australia and cannot get mobile reception at home.. I therefore rely on my home internet wifi to send imessages to my family with iphones. There is one family member that can send me an imessage but whenever I try and reply it always comes up green (mobile) and I get error. I have Send as SMS disabled IMessage is enabled Mobile Data is turned off also. This is so frustrating. Any help would be greatly appreciated