Optus keeps telling me my data usage is out of control when in reality, I hardly use my ISP data (use WiFi at home).

So, I need to manage what APPS are using my data.

Take control of how your iPhone uses its data connection.

Restrict iTunes and App Store downloads
You can prevent iTunes and the App Store from downloading music, movies, apps and so on when you are away from a Wi-Fi signal. To do so, go to Settings > iTunes & App Store and toggle off Use Mobile Data.

Disable background app refresh
iOS apps can update in the background, grabbing new content as they sit idle so they can show you the latest news when you return to them. Go to Settings > General > Background App Refresh in order to turn this setting off completely or WiFi. You can also manage APPs individually from the list.
See which apps are using the most data

Curious to know which apps are consuming the most data? Go to Settings > Mobile Data and you can see how much data you’ve used in the current billing period and below that you’ll see a list of your apps. Under each app’s name is the amount of data it has used for the current billing period. You can toggle off any app that you think is eating more than its fair share.

Disable Wi-Fi Assist
Wi-Fi Assist is a great feature where your iPhone hands off a weak Wi-Fi signal to your mobile network to prevent pages from loading slowly (or not at all) as it clings to the last remnants of a Wi-Fi signal. If you sit on the edge of a Wi-Fi network at work, say, then your mobile network may be assisting more than you’d like and running up data charges.

To disable Wi-Fi Assist, go to Settings > Mobile and scroll all the way to the bottom to turn off Wi-Fi Assist.

Download music, don’t stream
Streaming music or podcasts for long stretches when you are away from Wi-Fi can quickly add to your data usage. Most music and podcast apps (like Spotify and Apple Music) let you restrict streaming to Wi-Fi only, which will then force you into the habit of downloading playlists or podcasts before playing them instead of streaming them over Mobile Data. Let’s look at Apple Music and Apple’s Podcasts apps as examples.

For Apple Music, go to Settings > Music. In the Streaming & Downloads section, you’ll see two settings if the first is enabled.
The first, Use Mobile Data, lets you disable streaming via a mobile connection entirely. If that’s too drastic a measure for you, then you can leave that setting enabled and turn off High Quality on mobile to stream songs at a lower bitrate when you aren’t on Wi-Fi. For the Podcasts app, go to Settings > Podcasts and turn off Mobile Data. You can also enable Only Download on Wi-Fi to prevent podcast downloads from adding to your data usage.

Fetch mail less frequently
Check to see how frequently your email account is set to fetch new mail — the less frequently it fetches, the less data (and battery) you’ll use.
Go to Settings > Mail > Accounts > Fetch New Data. First, make sure Push is turned off if you want to save data and don’t need new emails pushed to you constantly. Next, see what the schedule is for Fetch at the bottom of the screen. If you choose Manually, then the Mail app will check for new email only when you open the app.

Use Safari’s Reading List
You can queue up articles while you’re using Wi-Fi to read later when you’re on a mobile connection or out of range completely.
When you add a page to Safari’s Reading List, Safari downloads it for offline viewing.
To add an article to the Reading List, tap the Share button at the center of the bottom navigation bar and then tap Add to Reading List. If you use iCloud, then it will share your Reading List with your other iOS devices, but you can stop it from sharing via a mobile connection by going to Settings > Safari and scrolling down to the bottom and toggling off Use Mobile Data for the Reading List feature.

Chrome recently added the same feature but it requires an extra tap. To add something to Chrome’s reading list, tap the triple-dot button in the top right, tap the share button and then tap Read Later.

Source: cnet.com

Turn Off Data
If you don’t want to turn off data outright, you can pick and choose which apps get to use it.

• Launch the Settings app from your Home screen.
• Tap Mobile Data
• Tap the switch next to each app for which you’d like to turn off Mobile Data usage. Just scroll down to view them.

Note: In the USA Mobile Data is referred to as Cellular Data

I hope it helps..