Mini won't send SMS messages. iMessage is A OK.

My Mini will not send SMS messages, except for the STOP and END messages responding to political senders.

My iOS devices work OK. If I respond to a friend on the phone, the post also shows up on the desktop version.

I've been over and over all the settings with Consumer Cellular and all is well with them. I've been over and over all the phone and Mini. Everything seems correct. I'm at my wits end. There must be something I'm missing. I was sending SMS fine until a couple of weeks ago. The problem might have begun when the latest Mac OS system update came through. I've reloaded the latest system. HELP!