How do I restore DND back on my iPhone after iOS 18.5 update?

DND still exists on the iPhone, it is just listed under Focus. When you go to Focus, you will see all the different Focus settings, and DND is at the top. You can still use it in the same manner that you did originally with DND. When Apple added Focus, it did not affect my DND settings at all. I had a daily DND configured to turn on DND at 10:30pm and turn back off at 7:00am. It worked flawlessly.

What exactly about DND is not working or is it that you just don't like the looks of the new settings?

Learn how to use Focus, including silencing notifications by reading this link --> Set up a Focus on iPhone - Apple Support

You cannot revert to an older iOS. Apple does not support downgrading iOS. You can find all of the directions in the link that lobsterghost1 provided. By default, everything should be restricted, but if you allowed things like emergency contacts in your DND, that would be allowed, but not anything else. It is quite easy to remove access to everything in the DND setup.