Originally Posted by Sant

In the past, when I purchased an International data pass, I'd enable "Airplane Mode" once my phone connects to the foreign carrier. I only disable it when I need to access the Internet and don't have access to WiFi.

When I charge the phone overnight at the hotel, I make sure to leave it on Airplane Mode. There had been times I wake up and noticed my iPhone 5S (on iOS 7.x) no longer connected to the hotel's WiFi.

As you own an iPhone 6 Plus, iOS 9's "Low Battery Mode" appears to do the same by disabling Automatic Downloads and Mail Fetch. It also disables Background App Refresh which would prevent apps from accessing the Internet unless you're actively using them.