I'm trying to do an iTunes backup for an iPhone, but iTunes only lets me "set it up as new" or "restore it"

My mother started using an iPhone 6s (running iOS 13.3.1) last week and now I'd like to back it up for the first time using iTunes (it's on an iMac running macOS Sierra 10.12.6).

However, when I connect the phone to the computer using the charger cable, it doesn't bring me to the page where you can see your iPhone storage usage (which is what I see when I plug my iPhone in), but it just shows the "Welcome to your new iPhone" page, and my only two options are to restore a previous backup on the phone or set it up as new.

How do I go to the page that lets me back up the phone? I've tried searching on the internet but I found no answer.

I hope you can help me with this.