Download iOS 15 with cellular data

Your iPhone 7 cannot use cellular data to update. You need an iPhone 12 or iPhone 13 with 5G service to do so.

You can use a computer, however, if you have one. If not, take it in to any Apple Store, and they'll most likely help you.

Update software on your iPhone or iPad with Mac - Apple Support

Update software on iPhone, iPad, or iPod touch in iTunes on PC - Apple Support

An iPhone 7 does not have 5G support so you cannot update your software over 5G

Download the update via a pc or Mac instead

Update your iPhone using a Mac with macOS Catalina or higher

On a Mac with macOS Catalina or higher, you will use the finder in order to upgrade your iPhone. Make sure to Back up your device before starting the upgrade process.

On a Mac with macOS Catalina or later, you will follow the following steps:

1. Open a finder window
2. Connect your device to your Mac via a lighting to usb A or a lightning to usb c cable
3. Select your device in the left sidebar of the finder window
4. click general, then click check for update
5. Click download and update.
6. If asked, enter your iPhone passcode

Update your iPhone via a Mac running macOS Mojave or earlier, or on a Windows PC

On macOS Mojave or earlier, or on a windows machine, we won't use the Finder to upgrade your iOS

Instead, we will use iTunes. Make sure to Back up your device before doing any update or upgrade to your phone.

1. Open iTunes
2. connect your device to the computer with an lightning to usb a or a lightning to usb c cable
3. Click device button in the iTunes toolbar
4. In the summary tab, click check for update
5. Click download and update, follow The on screen instructions
6. If asked, enter your iPhone passcode.

If you have no Internet connection save for your cellular data, then yes. Perhaps someone else would let you use their Wi-Fi?