iPhone 7 Plus touchscreen won't work!
Unfortunately, Apple is no longer signing iOS v11.4 so you cannot downgrade the iOS version. I've seen a few scattered instances of iPhone 7 touch not working on iOS 12. The common denominator seems to be aftermarket screens.

This is similar to the issue we had back when iOS 11 came out and lower quality aftermarket screens were also affected but on a much wider scale. You could contact Apple support or Store but they will probably just want you to replace the screen for an Apple original.

If you can't or don't want to do that, then go back to the shop that repaired your screen in the first place and see if they have an "original-quality" screen they can try to help troubleshoot the issue. Otherwise, you will have to wait until (if ?) this gets corrected in the next release.