This could be caused if there is no Internet connection. If the option to “Send as SMS” is turned off, the iMessage will not be delivered until the device is back online. You can force an undelivered iMessage to be sent as a regular text message regardless of the “Send as SMS” setting.2016-02-28
What does send as SMS mean?
Short Message Service
Do text messages go through servers?
“Text messages are such a primitive technology that's just unencrypted text going out over the network. It runs through servers and different carriers have different policies about how long they hang onto those messages.2019-11-12
Why are my texts being sent as SMS?
This could be caused if there is no Internet connection. If the option to “Send as SMS” is turned off, the iMessage will not be delivered until the device is back online. You can force an undelivered iMessage to be sent as a regular text message regardless of the “Send as SMS” setting.2016-02-28