I spent a good deal of the day wondering why I could not access my run app with my iPhone (I need to verify the responsive design). I also could not access the personal web server of my Mac. Then I installed Mamp on the PC to verify, and the same happened. The iPad was connecting just fine.
Wondering why that was, I looked again in the settings and it hit me : Cell data was on, and for some reason superseded the WiFi connection. As soon as I switched off Mobile Data, the app appeared.
Sigh 
Go to Settings->Cellular and scroll all the way to the bottom. There’s a new setting called WiFi assist. If your WiFi signal is unstable, this automatically switches you to mobile.
Oh. Right. That was discussed not long ago. It really is poorly implemented. The WiFi router is about six feet away from my desk, so Wifi is way better than the one dot cell signal. How come the phone does not revert to wifi when I come back ? I have switched off that pesky feature.
I see how that can be a big problem with WebSessions.
or phone bills 
If youre switching from a local DHCP address to an internet connection as a whole then that wont work 
But there is a bigger problem with WiFi assist in that even if youre connected to an IP address that is routable from both situations the session in the web app doesnt survive the switch. And yet it doesnt display any errors either, it just sits there looking like you clicked on something but not doing anything. I expect Apple will adjust those WifiAssist parameters in the next iOS update so that its not quite so aggressive but it will still mean that unless the session handling in the WebAddition is altered to allow connections from multiple IP addresses a web app will always drop out randomly sometimes. There is a feedback report you can join in on to help get that sorted out.
No doubt that WiFi Assist will be improved eventually, and hopefully such a ridiculous thing as losing access to a local area network web site won’t happen so easily, but the IP switching will remain, and that is a colossal issue with Xojo Web Apps.
Especially as this technique is bound to happen with Android as well.
Already, in France, the local carrier Free has implemented a solution like that with their WiFi hotspots.
I do not not know either what Google Project Fi carrier switching may imply too. In the current state of mobile connectivity, it would make sense that changing carrier may imply a change in IP as well. And it would be surprising if that technology did not become more commonplace.