My iPhone keeps telling me that it is backed up

You’ve apparently been backing up to iCloud using Wi-Fi, and there’s no Wi-Fi available right now,to,perform a backup, and your iPhone and your cellular plan supports 5G which means that cellular backups to iCloud are possible, but—as indicated by the message you’re showing—potentially costly depending on the details of your cellular plan.

If you are somewhere where think you should have Wi-Fi, you’ll want to sort out why your iPhone is not connected to Wi-Fi.

If you are somewhere without Wi-Fi and want to back up now, then select “use cellular data” and perform the backup now.

Details on backup options and alternatives: How to back up your iPhone, iPad, and iPod touch - Apple Support

Click -> Use Cellular Data.

Or click Cancel if you want to use Wi-Fi.