Hi Lawal19,
Welcome to the Apple Community.
If a setting is off, iPhone uses only Wi-Fi for that service.
Apple support have an article online that may help you check that cellular data is correctly turned on for your iPhone.
Says:
Connect iPhone to a cellular network
Your iPhone automatically connects to your carrierʼs cellular data network if a Wi-Fi network isnʼt available.
If iPhone doesn’t connect, check the following:
- Verify that your SIM is activated and unlocked. See Set up cellular service on iPhone.
- Go to Settings > Cellular.
- Verify that Cellular Data is turned on. On models with Dual SIM, tap Cellular Data, then verify the selected line. (You can choose only one line for cellular data.)
Complete article:
https://support.apple.com/en-au/guide/iphone/iphd1cf4268/ios
Also:
View or change cellular data settings on iPhone
Set cellular data use for apps and services
Go to Settings > Cellular, then turn Cellular Data on or off for any app (such as Maps) or service (such as Wi-Fi Assist) that can use cellular data.
If a setting is off, iPhone uses only Wi-Fi for that service.
Note: Wi-Fi Assist is on by default. If Wi-Fi connectivity is poor, Wi-Fi Assist automatically switches to cellular data to boost the signal. Because you stay connected to the internet over cellular when you have a poor Wi-Fi connection, you might use more cellular data, which may incur additional charges depending on your data plan.
See the Apple Support article About Wi-Fi Assist.
Complete article:
https://support.apple.com/en-au/guide/iphone/iph3dd5f213/15.0/ios/15.0
Hope this is helpful :-)