WiFi accessory connectivity issue with iOS 12
It is hard to say what was changed in iOS 12 as the list of changes is very long, and you haven't specified which equipment, you're using, nor which app you're running.

However, I can say that behaving apps have no problems with WiFi and cellular on iOS 12. As long as the app is written to standard, you can access both at the same time without any issues - on iOS 12 as well as earlier.

The issues you're seeing are most often caused by incorrect network settings on the WiFi accessory. Specifically if the WiFi accessory sends out DHCP replies with a default gateway set, it is doing so incorrectly, and that can and will give problems with using cellular at the same time.