Hi,
just had the same issue and solved it by changing my wifi
My network config is : one internet box (freebox in France) + one TP-Link routeur linked together via ethernet cable
my computer and my iphone are connected to the TPlink router because the wifi spped is definitively better
=> it was the issue
when I run the ionic serve => browser works fine via the local host
but any device iphone 5S and SE get the message unable to load the page (the request time out)
it was due to the routeur, I just switch my laptop and my iphone to the wifi provided by my internet box and everything is fine now 
I need to check is port 8100 is opened on y routeur, but seems not 
I send couple of hours to install / desintall / reinstall…
hope it will help few of you
sorry for the beginner way I describe my issue I coded html /php/action script 15 years ago, and start ionic since yesterday 
Br,
Flyhooo