Can only send MMS over cellular data

I am having a problem with sending MMS messages and imessages over WiFi. I am connected to WiFi all day and I know the connection is good because I have no other problems with other apps that require internet, My problem is that as soon as I turn on my cellular data (even if I'm still connected to my WiFi) I get all the group messages and picture messages I was waiting for. I can't send any MMS messaging or imessages on WiFi either. This is mostly a problem because my husband and I are on a limited data plan and I don't want to waste it all on recieving and sending MMS messages. I have made sure group and MMS are enabled in settings but not really sure where to go from here.. I have had several family members look at as well and we can't figure it out! Any suggestions would be very appreciated!