Why does my iphone 6 keep resetting?

I live in the US, but currently I'm in Australia for a couple weeks. I turned off cellular data(so the corner of my iphone always says no service) but I'm connected to Wi-Fi a lot. But whenever I'm off Wi-Fi my phone resorts back to factory settings. First the "hello bonjour konichiwa" screen shows up. Then I have to sign in to my apple id, but it does not let you sign in to anyone elses account. How do I stop the phone from doing this?

Turning off cellular data does not mean you would see "No Service" where the carrier name usually is. That just turns off your ability to use internet from your carrier, which is sort of a null point in this case since you are nowhere near your carrier's cell towers anyway.

Your phone saying "No Service" is because of your location relative to your carrier's network, not turning off cellular data.

As far as the phone resetting that's a new one. I don't see why using Wi-Fi or not would cause the phone to reset. Don't see why you'd need to sign into someone else's Apple ID in any case.

Usually when out of the country for a relatively short period of time (weeks, as opposed to months or years) it is advised that you turn off roaming and/or use Airplane Mode to avoid expensive roaming charges. You'd see a plane icon where you are seeing "No Service".