I can’t update it with my mobile network

That is correct, with an iPhone 7, you need to connect to a computer or WiFi to update. you can only update over cellular with an iPhone 12 or higher and a 5G connection.

Update your iPhone, iPad, or iPod touch

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.7

• If you have an iPhone 6S through 7, or SE 1st generation, you can update to 15.7.8

• If you have an iPhone 8 or higher, or SE 2nd or 3rd generation,you can update to 16.6.1

Go to Settings > General > Software Update.

This link will show you how to update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone or iPad - Apple Support

This link will show you how to update using your computer -

Update iOS on iPhone - Apple Support

I want to update my iPhone 7

Then, by all means, do.