I may have the solution.
I have had the same problem: no alert sound or banner when receiving text messages on IOS 14 when the display screen is dark and locked. I went through all of the suggestions in the settings: checking Do Not disturb, checking sound settings, restarting, and other items. None worked.
Then one day my wife and I were testing the alerts we decided to "Swipe Down," to view any notifications. It did not show any text notification, BUT, there was a notification stating that all notifications would be silenced until "Do Not Disturb is Turned Off." Then we touched that message, and a box appeared underneath that to "turn off the Do Not Disturb." We hit the box and it turned off the "Do Not Disturb." Oddly I had already checked in Settings and made sure that I did not have "Do Not Disturb" turned on. But we clicked the box anyway, and it worked. From that time on, all of my alerts are working. Very strange.
It seems that, somehow, the "Do Not Disturb" is screwed up in IOS 14 making it appear to be operating even when you turned it off.
So, test your messages by swiping down and this may work.