Have you tried going into your Settings, Messages… and do either of the following; if IMessages is toggled to on, go further down to SMS and ensure it is also toggled to On… this will help ensure that if a message can’t be delivered by IMessage it should be delivered by SMS. Another option to test is to Toggle IMessage to off, and ensure both SMS and MMS are toggled to on. This would force all messages to be sent by SMS/MMS…however keep in mind there are size limitations to your messages sent this way! Good Luck.