Why can’t use my data for update

You don't mention what device you are using, or which iOS version you are trying to update. However, my iPhone XS Max has offered to let me use cellular data (at least in share to support slower with WiFi) for at least the last 2 versions. If you are not getting this it could also be restricted by your network/carrier and blocked by the more restrictive and tight fisted.

See -> Use 5G with your iPhone -> Use 5G with your iPhone - Apple Support

Here update SE 2nd gen with 4G.