This is very strange. Sometimes when I send a text is shows not delivered. When I click on the text it gives me and option to send as an sms after I click it the text is sent. Why does it do that ?
This can happen on occasion. Simply resend it. Also, I find that text messages generally send more quickly when using the LTE network, so I would try using that network if you're currently using the HSPA network instead.
This has happened to me before, too. It only happened once per contact, though and not with every contact. When I pressed "send as SMS", it didn't happen again after that. I figured out that it was only happening with people who had a certain type of Android phone. So it was as though before I switched to Public Mobile, we were communicating through a different avenue of some kind. Easy fix though, just hit "send as SMS" and it shouldn't keep happening with the same contact.
Try to keep your data plan on.
Mostley because your data plan is off and your phone tries to send SMS messages true the WiFi connections which not talking properly with public Mobile MMS setting.
When you are in your WiFi area you are auto connect to your WiFi router if you choose to connect auto,
To check if its auto connect just hold your finger on the name of connected WiFi and check information.