There is an option in the Apple Messages settings to enable the app to use SMS when when messaging. Usually this is enabled by default, but once turned on, Apple Messages will automatically send using the SMS protocol when unable to use...
Part 1 of 3:
Enabling SMS in Apple Messages
Open the Settings app.
Tap Messages.
Part 2 of 3:
Forcing a Message to Send as SMS
Open the Settings app.
Tap Wi-Fi.
Tap the Wi-Fi slider to disable. Green indicates the feature is on while grey indicates it is off.
Tap Settings. This is in the upper left corner and will take you back to the Settings menu.
Tap Cellular.
Tap Cellular Data slider to disable. Green indicates the feature is on while grey indicates it is off.
Open Messages.
Tap a chat.
Update 04 March 2020
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