Suddenly I was not able to send SMS messages via iMessages. Tried every possible troubleshooting option, with only 1 working. That was turning of iMessages itself under messages.

However this was not good enough. It disabled internet chat/continuity on my macs and pads etc. So I had to turn this back on.

It took me ages to find this but it was quite simple in the end.  If your SMS’s are getting stuck. Just click the SMS message that either says ‘failed to send’ or stuck in the sending mode and select ‘Send as SMS’.