I had the same problem but I figured it out. I was using the normal half moon button for Do Not Disturb... but the auto reply only recognizes it’s supposed to send the auto text when it’s in the Do Not Disturb While Driving mode. Go to settings>control center>customized controls then scroll down to Do Not Disturb While Driving and add it to your control center. Now exit out and flick up from bottom of screen and you’ll see an icon with a car. Click that and it will put your phone in Do Not Disturb mode and activate your auto reply message. I use this feature when I sleep and costmized the message accordingly. I also specified I only want to send the auto reply to my favorites. Hope that helps!