Can you back up data only on WiFi, on an iPhone?

IMazing <https://imazing.com> will backup over WiFi to your Mac for free. You would pay for additional features, but a basic backup is free.

If you’re asking about backing up to iCloud, other than Wi-Fi you can backup to iCloud over cellular. This will likely consume a lot of your cellular data, but it can be done. Look in your iPhone’s iCloud Backup settings, there is a “Back up over cellular” option that you can enable.

How to back up your iPhone, iPad, and iPod touch with iCloud

I have a few thoughts to share with you which may be useful. (1) it is important to know that using iCloud does not back up data, although it does have some back up features. iCloud mirrors what you have in your devices for cross platform sharing. (2) To back up your computer, you should back up to an external device. You can use an app such as Apple's Time Machine to do that, or you can even back up yourself manually. (3) I personally would not use Wi-Fi for backing up. as it is a bit slow. I think it's better to back up to a wired device. It's more reliable.