After some hair pulling I figured out that there's a simple fix if you iPhone is failing to send texts specifically if roaming (no data available) or your data is turned off to conserve bandwidth, and you're not on wifi either. iPhone seems to come with a necessary toggle turned off. Just go to Settings, Message, and toggle "send as SMS" to On. Otherwise it will try to use iMessage over wifi or data only. If you switch this toggle on, as it explains clearly, it will switch to regular cell SMS when iMessage isn't available.