WiFi accessory connectivity issue with iOS 12 I have a WiFi to Serial accessory that I'm using with an iOS app.

I was able to use it properly until iOS 12.

The app requires both Wifi (to collect data from the accessory) and Cellular (to send data to the cloud) to work. I can get either WiFi or Cellular to work separately but not at the same time except that I can access the accessory settings page over http with Safari (but not with the app over TCP). The app is using GCDAsyncSocket library for requests to the accessory.

I've been able to get things work randomly by disconnecting the WiFi network, reconnecting it and set set WiFi assist on/off but I think that there should be a way to get things working properly.

What exactly changed on iOS 12 to get things broken? 

Thanks
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.