How can I download and install software update on my iPhone without connecting to Wi-Fi?
iOS needs a Wi-Fi connection to download and install software update. It is not possible to download the software update over mobile data, irrespective of speed or data cap. This is by design.

If you do not have a fixed line connection with Wi-Fi access point, you can create a Wi-Fi hotspot on one of the other mobile phones.

Alternatively, if you have access to a Mac, you can download and install the software update via iTunes and update the device.