If you plan to travel outside the Philippines, this is a helpful tip for you. I discovered this painfully after a recent 17-day business trip last March. I was charged expensive fees for data services even when I was only using the internet when my phone was connected to the hotel's wifi. 

You know how Telecom providers auto subscribes you to unlimited-data-for-a-day promo for a lesser fee? It gets renewed every day too. For Smart, it is called SURFABOARD 550. I have always wondered why I catch my phone using data even when I have explicity turned off Cellular Data settings. I learned that it was because my Wi-Fi Assist was ON. According to Apple's support site, Wi-FI Assist means:

How Wi-Fi Assist works

With Wi-Fi Assist, you can stay connected to the Internet even if you have a poor Wi-Fi connection. For example, if you're using Safari with a poor Wi-Fi connection and a webpage doesn't load, Wi-Fi Assist will activate and automatically switch to cellular so that the webpage continues to load. You can use Wi-Fi Assist with most apps like Safari, Apple Music, Mail, Maps, and more. 

When Wi-Fi Assist is activated, you'll see the cellular data icon in the status bar on your device.

Because you'll stay connected to the Internet over cellular when you have a poor Wi-Fi connection, you might use more cellular data. For most users, this should only be a small percentage higher than previous usage. If you have questions about your data usage, learn more about managing your cellular data or contact Apple Support.

That means when my phone was connected to the hotel's wifi and when it had experienced poor connection, it automatically turned my Cellular Data back ON. To stop that behavior, simply turn off your Wi-Fi Assist setting. This is how.

Using your iPhone, Go to Settings app then Cellular > Cellular Data OFF. While you are still in your Cellular screen, scroll way down (almost at the bottom). Find Wi-FI Assist then OFF. Refer to screenshots for better illustration.