How do I roll back my iOS version? The 12.2 update has broken my WiFi
Basically, you can't.

Apple stop signing previous versions shortly after a new one comes out. iOS 12.1.4 is no longer signed since April 4.

Ref: MacRumors - Apple Stops Signing iOS 12.1.4 Following Release of iOS 12.2

I would look elsewhere for your solution, because had it been a widespread issue it would be well reported by now.