Help SMS using wrong SIM

My Moto G (2014) has suddenly started to try to use the 2nd SIM when sending a text. This fails because I have the first SIM set to provide data (the phone won't allow data over both SIMs simultaneously). I have also set the phone to use the first SIM for texts, yet it is not doing so.

In case anyone is wondering why I don't just use the second SIM, I have no coverage on that SIM at home (where I was trying to send the text) and it charges more per text.