A question: is automatic downloads enabled?, if so see -> How to manually update apps on your Apple device
How to manually update apps on your Apple device - Apple Support
Check also which apps are using data, see -> Use cellular data on your iPhone or iPad
Use cellular data on your iPhone or iPad - Apple Support
Check also the Wi-Fi connection, see the following Apple articles:
Connect to Wi-Fi on your iPhone, iPad, or iPod touch
Connect to Wi-Fi on your iPhone, iPad, or iPod touch - Apple Support
If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network
If your iPhone, iPad, or iPod touch won't connect to a Wi-Fi network - Apple Support
Recommended settings for Wi-Fi routers and access points
Recommended settings for Wi-Fi routers and access points - Apple Support
If you need help with your Wi‑Fi password
If you need help with your Wi‑Fi password - Apple Support