How to download iPhone update without Wi-Fi?

You can use a computer if you don't access to Wifi and use iTunes (Windows) or Finder (Mac).

You can go to a friends house and use their Wifi

You can go to a place like Starbucks and use their Wifi

You can't use cellular data on an iPhone 11, if you don't have Wifi

Ideally, a friend might let you use their Wi-Fi at their place for 20-30 minutes.

Otherwise, take the phone down to the coffee place or bookstore and use the Wi-Fi there. If you have an Apple Store nearby, you can use the Wi-Fi there as well.

Contact your carrier and ask for a Jetpack or MiFi or equivalent device.

Might also want to ask your carrier how much it costs to download the amount of data required for an iOS update, too.

Otherwise, see the answers in the replies above; find a Wi-Fi network.