I'm working on a little WiFi web server with the Onion Omega, not connected to the internet.
I've tried some different smartphones, Samsung, Oppo, Apple, OnePlus...
I tried 3 models of OnePlus, they all cannot reach the web page when the onion doesn't bridge the WiFi hotspot to the internet.
Most phones test the connection, and will reject an access point if they cannot reach internet. If my memory serves me right, there is a "fakeinternet" package to solve this.